Based on a bit of research, reading articles by 343i, as well as looking through their job vacancies, I think these would be pretty close to the right answers, and 343i are already trying to solve some of these issues - unfortunately it’ll take some time. I’ve broken the answers to each of your questions into sections:
- The Graphics In General.
- Ray-tracing (RT lighting and reflections).
- Special Effects.
- Draw Distance and Optimisation.
- Slipspace 2.0 with Ray-Tracing is coming!
1. THE GRAPHICS IN GENERAL.
If you read and remember the forums in 2015 onward, many long-time Halo fans had issues with Halo 5’s graphics such as “too glossy, shiny and new looking”, “too much geometry detail”, and “too bright and colourful”. As I said, when Halo fans say “Give us the Halo artstyle we remember”, what they really mean is “Give us designs like we remember from the XBox 360 days”. That was part of the feedback they took when designing Halo Infinite.
As 343i’s Director of Art Nicholas Bouvier (a.k.a. “Sparth”) said “We always knew it was going to be a challenge to balance between the simpler designs of Halo 1 and 2 and the requirements and expections of Next-Gen.”
After all, how do you make “blockier” graphics like “the good old days on XBox 360”, when to unlock Next-Gen hardware we need new tech things like UE5’s “Nanite and Lumen”:
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“Nanite” as “unlimited geometry” (accelerated by the the new Mesh Shaders geometry hardware in the new XBox Series S and X), so instead of 10K-100K triangles, new tech easily allows 100 million triangles or even billions of triangles culled down to 20 million onscreen to make a scene.
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“Lumen” for dynamic lighting and global illumnation (and which can be accelerated by the ray-tracing hardware in XSX and XSS and next-gen PC).
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instead of simply aiming for 4K picture quality, the UE5 Demo was using upto 8K textures on 6K geometry “all running on a PS5”.
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we also need SFS (Sampler Feedback Streaming) running on the next-gen SSD (an NVMe drive), because it decompresses the compressed textures “on the fly” for high-speed streaming into the Graphics. The new XVA Format (XBox Velocity Architecture) also only needs to deliver the “bits” of the textures we actually see onscreen, not the back, bottom nor unseen sides of the textures, so they take up less memory allowing them to include far more high quality textures in pictures.
Whether the Halo Community would accept this truly “next-gen Halo” is an issue. Anyone up for a photorealistic version of Halo:CE’s “inclement weather” done UE5 style and using the power of “next-gen” console graphics - looking something like this?
https://www.youtube.com/watch?v=kT4iWCxu5hA&t=56s
2. RAY TRACING (RT LIGHTING AND REFLECTIONS).
Aside from catering to that “classic artstyle” that many Halo fans seek, the other half of the issue is to do with timing of 9th gen hardware and software launches and efficiency in games development.
NVidia only revealed what would become the “next-gen” of hardware with ray-tracing in 2018. The next-gen console with ray-tracing hardware released in November 2020, but look at the timetable for launch of it’s software to “switch on” the new next-gen hardware:
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May 2020: Microsoft release the launch version of DirectX 12 Ultimate with the ray-tracing command-set in it to activate the next-gen hardware. This would give 343i only 5-6 months to do the ray-tracing upgrade as well as apply it, when they need to get the game out the door.
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June 2020: NVidia release their DX12 Ultimate driver for their RTX 2000 cards.
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November 2020: AMD & Microsoft launch XBox Series S&X, RX6000 series gaming graphics cards for PC and their DX12 Ultimate driver to use the ray-tracing and mesh shader hardware, all at the same time 343i were supposed to launching Halo Infinite - zero time to use the new DX12U hardware and software!
So, at launch, it was impossible for Halo Infinite to be a “next-gen game” with ray-tracing made for 9th gen consoles because the hardware and software only launched so close to the end of the game being developed - less than before launch - that’s why it’s an 8th gen game made for XB1, XB1S and XB1X, and which could be “Optimised for Series X&S” to add in faster disk loading, higher framerates etc., but not the “next-gen” graphics of DX 12-U, or the full-on “next-gen only” DX 12.2 which though the consoles are ready for, it’s still in beta testing and hasn’t officially launched yet. “Unfortunately” 343i are “mere game developers”, not Time Lords from Gallifrey like Dr. Who who can travel in their Tardis to the future when all the hardware and software is ready for a true “next-gen Halo Infinite”, build it into their Engine, and deliver it all back into 2021 just in time for launch! 
3. SPECIAL EFFECTS.
I made another post about this this recently, but 343i have had job vacancies for Special Effects Artists dating back into last Summer, alas they were not filled before launch, hence the SFX may not be upto the same standards as other perts of the graphics. Since then, the FX Artist job has now been filled, but the Senior FX Artist job vacancy is still open after six months, whose role amongst other things involves:
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Create all types of photorealistic, high quality effects for weapons, vehicles and characters.
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Concept and prototype new effects as well as raising the quality bar on existing content.
See the full job advert here:
https://careers.microsoft.com/professionals/us/en/job/1101017/Senior-Fx-Artist-343-Industries
So, even long before launch, it seems 343i themselves already agreed with you that some of the Special Effects could be a lot better, so when the job is filled, “raising the bar on existing content” hopefully means updates for Halo Infinite to improve the SFX you’ve described.
4. DRAW DISTANCE AND OPTIMISATION.
Things like Draw Distance and background texture quality and amounts of pop-in partly depend on the hardware you’re using. You didn’t mention the power of your PC, and speed of your disk - are you using a hard disk/SSD/NVMe drive? Nor did you state what resolution and graphics quality (Very High?) you’re playing at, whether you’re going for high framerates etc. It would be easier to comment if you stated that.
In general, framerate and other stuff also require MSI Afterburner and Riva Statistics Tuner to be completely off and not running, as they interfere with the game.
5. SLIPSPACE UPGRADE FROM VERSION 1.0 TO 2.0 NEXT-GEN ONLY IS COMING!
Development efficiency means “get the game out the door” on deadline, making the game for XB1/S/X where most Halo players are right now, then worry about adding next-gen features for Series X/S and ray-tracing hardware (NVidia RTX 2000 or AMD RX6000 or above).
343i have hired an extra Senior Lighting Artist to assist the one they already have and who’d came to 343i from Naughty Dog after finishing TLOU2, which has some great lighting, and the new hires had to have experience of ray-tracing.
Part of their job is to work with the Engine and Technical Art Teams to help upgrade Slipspace Engine with DX12 Ray-Tracing, working out a balance of desired lighting effects and improvements whilst keeping an eye on hardware workloads and game performance / framerates.
Once this Slipspace Engine 2.0 is done, not only can Infinite’s “Campaign 2” (a.k.a. a “Halo 7 campaign”) be built with ray-tracing and other next-gen features “from the ground up”, but they can finally add ray-tracing to the original Halo Infinite campaign that just launched too. 
I hope this helps answer some of your questions. 343i are aware of the complaints about the issues, and have been trying to deal with them, but they have a bunch of open job vanacies they’re still trying to fill. Then, as Nicholas Bouvier said, they also have to wrestle to balance their XBox 360 “Retro Artstyle” with the next-gen tech.
Hopefully, improvements to SFX and Ray-Tracing for next-gen consoles and PC will be added later this year, along with further optimisations, perhaps improvements for pop-ins etc.
Hope this helps.
