I was not able to reproduce the issue at all in singleplayer using just Pixelmon and a standard set of performance mods (the same ones that I was using when the crash happened on a server) so I also started up a dedicated server to test if the issue was caused by running in multiplayer - this was able to reproduce the issue almost instantly. (As a side note, I was also able to confirm that quickly killing the instance in MultiMC prevents it from crashing my computer entirely, and it "unlocks" when the instance closes - I also noticed that audio continues to play from the instance when it freezes, and this continues until Windows eventually locks up completely, so the game is still running fine just unable to update visually)
Testing with just Pixelmon on both the client and the server about 15 times, the issue did not appear, so I added back the performance mods one by one (only on the client, the server never had any extra mods even when the issue was occurring) - the issue seems to occur when the Helium performance mod is installed on the client, even if it is the only mod installed besides Pixelmon itself . I'm attaching the server and client's latest.log and debug.log (there was no crash report to attach, since the game itself doesnt crash, only freeze) both with verbose turned on and off on both ends: https://drive.google.com/drive/folders/1dVdXT6DVqrZrU0aoE0-YzJJg6FFcDKEF?usp=sharing
I also tested a much more complete set of mods with multiple performance and content mods, the original pack I was using when the issue started, and removing Helium seemed to fix the issue completely.
TL;DR - Helium on the client appears to be the cause, using all the other performance and content mods I was using when the crash first happened seems fine