Got my desktop set up again, and the issue is still there. I did some troubleshooting, so here's all the info I could come up with:
3 monitor setup, not identical ones (game monitor: https://www.amazon.com/dp/B0BCK1K44F?ref=ppx_yo2ov_dt_b_product_details&th=1)
computer internals are the same (Ryzen 7 3700x, RTX 4070 Ti, 32GB RAM, game and windows are installed on (separate) NVME drives.)
windows 10
modpack and graphics driver updated as part of troubleshooting (no noticeable effect)
pretty sure the list of mods hasn't changed
GPU and CPU temps haven't gone above 80C so no throttling is happening
I've tested if win10 game recording matters (it doesn't)
when the FPS drops, GPU usage drops as well (lower FPS=lower usage)
today's testing was with a slightly laggy replay (not nearly as bad as the one I posted a while back)
Standard graphics: maxed out (for standard), dropped to 190 or so FPS at worst, usually 250-300fps, frontline still gets down to like 50fps (but that's frontline lol)
Improved graphics: also maxed out (prior testing showed this doesn't matter), could hold ~150fps with 99% gpu usage, dropped down to 30-60fps
I didn't bother testing with safemode today, but I figure it's pointless because we already know it's mod-related.
Guess I'll be playing at 4k 144hz.... in standard graphics LOL
EDIT:
turns out that replay was less laggy than I thought. I played a match on standard graphics (not in frontline) and it dropped down to 38 at one point.
It was mostly able to hold 144fps, but having the tanks.gg armor viewer open seems to drop it by like 10-15fps?
I don't always have that page open so I know it isn't the main issue, but that's specific enough maybe somebody can figure out what's going on?