Best settings for high quality YouTube animation?

I'm working exclusively with animation, never with camera footage. My unique needs are as follows:
- As accurate colours as possible
- No artifacts/grain on solid colours
- Can be uploaded to YouTube and still retains all of the above

Some preferences that aren't a requirement:
- Faster than VEGAS two-pass
- (new addition) a reasonable file size

I've been having issues with my preferred stock VEGAS codec, MAGIX AAC/AVC MP4, Internet UHD 2160p 59.94 fps, with it adding large blocks of artifacts on areas that should be solid colour. See it happening here. I'm looking into Voukoder to hopefully kill this issue for good. Problem is I can't figure out how to get a high quality render that stays high quality once uploaded to YouTube; most common issue I have is it gets super grainy, something not present when the file is viewed directly (I'm aware YouTube reencodes stuff you upload, I just can't figure out why the grain only appears with renders made from Voukoder).

I don't care much for render/upload time, I just need something that looks good. Not to say I need it 100% lossless, I just don't know where the compromise would be.

Any help appreciated.

Thanks for the reply, I tried ProRes422 first and - yikes that's hard to work with when viewing this massive file goes at 6fps max. Happy to have a benchmark that I know works in full quality if needed, but I don't think it'll work all too well in my workflow.

Strangely, after plugging in the settings I deemed necessary from the top comment on that Reddit post, the resulting video almost wouldn't play. It opens, audio plays, but video is stuck on the first frame. Also won't work for my workflow considering I need to like. Be Able To See It.

I'm viewing through VLC and I've never had issues like these before so I don't think this is an issue with my hardware/software or something.

Good news is these settings do look good, from what I can see, so it's a start!

I had selected the ProRes Alternative h264 preset and then applied the following to make the good looking but not playing video:
opencl=1 preset=slow qp=0 rc=cqp x264-params=aq-mode=3:aq-strength=1.000:keyint=180:min-keyint=180:bframes=8:ref=3

I figured something out that works, though I don't quite understand how it works so if someone could shed some light on this that'd be great:
1) Export from VEGAS, built-in ProRes HD preset
2) Send to Adobe Media Encoder, typical h264 preset, 35k bitrate as recommended
3) Result looks great, upload to YouTube and it still looks great

For some reason this process works totally fine when done with Media Encoder, but through anything else (namely Shutter Encoder, my preferred program) the artifacts are created.

I'd prefer to not use Media Encoder. I'm fine with having to do the ProRes render and then reencode, but I need to figure out what's different between Media Encoder and the others that makes the former work and the others fail. As far as I can tell every setting is the same, so I'm at a loss.

This doesn't really relate to Voukoder anymore (unless I could use it in some way here?) but I'd appreciate any help I can get, searching for this sort of thing is hard.

Correct. See, at least to me, Media Encoder makes it difficult to tell exactly what it's doing; the only thing I've changed from its default h264 preset is the target bitrate. I also don't have on-demand access to Media Encoder (the reason I don't want to use it haha) so I can't go back now and double check the preset's params, I'd need a day or so, but even then I'm not sure I know what settings are pertinent to this discussion. Afaik Media Encoder won't give me a string like the one I have in the third post - though I could just be oblivious to where to access it.

The working preset from Media Encoder is H.264 - Match Source, High Bitrate.

I decided to bite and try out exactly what the post you shared said, with Handbrake, and this result also got decimated by YouTube (worse than stock VEGAS export)

Seems like the only thing that works is whatever Media Encoder is doing behind the scenes. This is the output settings as described in Media Encoder:
3840x2160 (1.0), 59.94 fps, Progressive, Hardware Encoding, VBR, 1 pass, Target 10.00 Mbps

I've tried mimicking this in the other programs but it's just not enough information. Granted I don't know what the (1.0) is.

I keep trying with any combination of settings and it always comes out worse than stock VEGAS. I'm at a loss.

Took me WAY too much time but I figured it out, sort of, and figured I'd come back here to close it out: YouTube says on its recommended encoding settings page that it prefers and uses the BT.709 colour space in its videos. I've never heard of this one in particular, but I found that rendering to Rec.709 produces a result without any artifacting. No other setting matters - as long as YT doesn't have to convert the colour space (or does so very lightly) artifacting won't be a problem. It is exactly what I was asking for: everything is the same as what I've been using, but with one singular, pesky setting changed under the hood that makes YT retain quality after reencoding. Funny how that works, huh?

TLDR: use the Rec.709 colour space. That's it. Thanks all who helped me noodle around to find this.