First off, thank you for the quick and thorough reply.
The plugin bridge has always been nebulous to me but I am fairly certain(?) I'm not using it with any VSTs currently. All plugins that I use are 64-bit and I am using the 64-bit version of OpenMPT. Although I could be misinterpreting how the plugin bridge works.
The song I'm working on right now has 90+ instances of Kontakt and a few other VSTs. Kontakt in particular can get pretty bogged down with Event FX. Reverb and pass filters seems to hit playback the hardest, and adding timing envelopes to those as well, things have the potential to get out of hand pretty quickly. I'm not sure how much of an effect that many instances can have when only 30-40 are playing at a time, as it seems to really only have an effect when the more taxing instruments are playing.
Some instruments, especially in the more egregious Kontakt libraries, can have a pretty nutty amount of effects going on all at once before I get the desired sound and thus are probably taking up WAY more resources than a plugin might normally. My use-case for OpenMPT might be a little specific in that regard lol. But this might also mean that I should be looking towards Kontakt support. For what it's worth, Kontakt does have a multithreading option, and I've allowed it to allocate every thread I have, and the stuttering and this is the exact moment where a light bulb turned on and I realized that disabling multithreading support on Kontakt might alleviate the issue entirely. I even set the latency all the way down to 10ms. No more stuttering on playback.
It seems having differing settings when it comes to the processor between Kontakt and OpenMPT was what caused the issue for me. Looks like I've been saved from buying a new processor. Or maybe I've just lost the justification for buying a new one lol.
Thank you for your help! Hopefully my experience here will help somebody else down the road dealing with a similar issue.