Video exports with pixels no matter what settings i choose

I really need help, i've got less than a month to export this video and i've been trying for nearly a week now. Every time I export, certain frames turn out extremely pixely compared to the preview.

This is what a few of the frames look like in premiere pro.

Even the most "grainy" parts of the video look considerably fine compared to what exports when i try to render the video to an MP4 file. This is what the MP4 looks like, at close to the same frames (and this is repeatable).

On the last one, it looks pretty decent and close to the original. However, on the more "grainy" parts of the video, the export becomes extremely pixely and many times grainier than before. Along with this, at one point the audio was exporting sounding like it was coming from AM radio as well, but that problem seemed to disappear while I was troubleshooting. I still can't fix the video quality issue though and i've looked everywhere for answers.

I've tried HEVC (which I used for every other video I've been making and never had a problem) and I tried H.264 as well. I've also tried exporting using the previews at 1080x1080 (the video is 1440x1440) and that didn't change anything. I've also changed the preview format to Microsoft AVI at quality level 1.00 (so I could changed the preview resolution to 1440x1440) and that still did nothing. All these times, I also had "Maximum Render Quality" checked, "Maximum Bit Depth" checked, and these below settings (also tried software encoding):

I tried to change the CBR from 10 to 20 after reading that 20 is the minimum of what you need to have a decent 1080p export. My maximum is 20, so I don't know if this is a problem.

Lastly, I never changed the audio settings because they were at the highest:

Somewhere along the line, after changing some video related settings, the audio no longer is ruined (the high end was being destroyed as if there was lots of lossy compression going on) which doesn't make sense to me, but I'll take it. All I need to know now is how to fix the video, please can someone help 🙏🙏 thank you in advance

DJ D