Sorry for all your M1 issues. I have a similar machine here in my home office. I have heard others similarly complain about the performance. I hope I can provide you with some advice that may bring some relief.
I would first download the most current version of Premiere Pro, version 22.3. It is not available everywhere yet, so it may take some days to reach your Creative Cloud application. Look for it. I believe that engineers fixed some bugs related to M1. After downloading and before testing, be sure to delete any media cache with all projects closed so that you can delete all of it. As you download the new version, keep existing copies of Premiere Pro on your hard drive if you wish.
The anomalies you see on export are when you export to H.264. Is that correct? If this continues and you have a graphics-heavy export, you might want to choose a different format, like ProRes 422. With that master file, make your H.264 and HEVC copies for distribution. I believe you can work around your problem if you do so. Graphics heavy sequences seem to perform better in Macs with a dedicated GPU, like in your iMac.
The M1 operates on graphics using different tech, which might be the underlying issue. The M1 MacBook Pro has roughly the same performance capability as an NVIDIA GTX 1650 Super, which I believe offers 4 GB VRAM. If you require more VRAM than that for GPU accelerated effects, the Mercury Playback Engine, and hardware decoding and encoding of H.264, the M1 might not be performant enough for your taste. I am not sure about the iMac model you are using, but if it has more than 4 GB VRAM, it would be understandable that the iMac could get the job done more efficiently.
For general performance, check out memory preferences and allocate most of that 16GB RAM to Premiere Pro. Allow a few GB for macOS to operate, though. M1s have a known issue of requiring more RAM. You can check out the "Known Issues" doc for more info.
As far as MP3s go, it is not a great idea to use them in video post-production. Most editors prefer to handle .wav or .aif files as they have no compression. MP3s are pretty unreliable. Can you transcode the originals to .wav? That might work better.
I hope my response helps. Please return with questions.