I finally gave up a few hours ago and uninstalled, made sure to clear leftover registry keys and everything before reinstalling, and I haven't had a disconnect since (knock on wood). But before that I did the following things, in order:
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Checked pso2.exe network activity while questing and while in lobby to see if any packet loss or hiccups were happening on my end before disconnect (0%, never saw any issues)
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Reset my network stack in cmd and restart, disconnects did not improve
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Turned off IPv6, to no avail
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Finally wiped my install and redownloaded using Tweaker as before, which worked.
I hate that I'm recommending reinstallation of a ~90gb game, but if you're having this issue and the usual troubleshooting doesn't work for you, it might be time for the nuclear option.