Hmm, weird. I just tried booting (the same, unaltered from before) live session without changing the default boot options (not with nomodeset, just the pre-existing 'quiet, splasht, nosplash'), and it loaded everything just fine, including properly detecting my display resolution and everything. When I went into MX boot options while logged in though, it had 'quiet, nomodeset, splash' instead of what was listed in the live session startup screen (I guess it's just different since it's live?)
In any case, strange that it all of a sudden doesn't require nomodeset anymore and properly detects everything. I just wish I knew how to get my main install to mimic that behavior :/
edit: I seem to have fixed it on the main install (temporarily, at least) by adding
to the boot parameters; at the very least, it's properly detecting my display resolution at 1080p now. Would an acceptable solution be to just add nouveau.modeset=1 as a permanent boot parameter in boot options, or is that more of a bandaid fix that doesn't solve the underlying issue?