"Art by Demibowties[deviantart.com]

This might be the most stressful artwork I've ever worked on, partly because of the compression struggle and mainly because of the FPS synchronization.

APNG is a no-go for this particular artwork, the size was too ginormous. Back to the good old ♥♥♥♥ GIF with its compression then. Instead of increasing the compression or decreasing the color, I tried lowering the FPS to (which is this one). Perfectly acceptable result.
Except I was dumb enough to realize that the background animation is 29.97 FPS, so the artwork will play faster than the background. In case anyone reads this and wondering, "1 second is still 1 second regardless of FPS, how can it be faster?" It is because GIF and APNG are not a video format, therefore they rely on frame time delay. This leads to the next problem.

Note: I don't want to use workshop showcase as it has the bulky top side that will basically take up the background quite a lot.

If that's the case, I can only render it at 30 FPS (rounded up from 29.97), just gotta ramp up that compression. But no, they still don't sync, that's weird. I went to use an online converter to convert from GIF to MP4, downloaded it, and checked on the FPS, it was 33.33 FPS.
Oh wow, why would Photoshop render 30 FPS input to 33.33, that's weird? I did a long ass math calculation (since I suck at it) and found the answer. Here's the breakdown:

Artwork Duration: 5 seconds
AEP Artwork FPS: 30 FPS

How did I get 33.33 FPS instead?
Answer
Again, APNG and GIF are not videos, they rely on time delay per frame. Since I didn't have enough frames, Photoshop has to do something with the remaining milliseconds within the 5 seconds duration and it just increases the delay to 33.33 ms.

Oh ok, I get it, I just have to increase the frames to make the time delay 30 ms per frame. I changed my project to have 33.33 FPS instead because it has 167 frames. In comparison, I did a quick check below:

All check, that's the closest thing to 30 FPS, FINALLY.

But, no it still couldn't sync even for the first 5 seconds. Let's say even if it was because of caching problem, It will eventually go out of sync anyway after some time without refreshing. GIFs will never be able to sync with a 5-second 29.97 FPS video. I'm dead inside.

In conclusion, rather than uploading a heavily compressed 30 FPS artwork, I'd stick with the less compressed 24 FPS."