The 2TB drive arrived yesterday, and just before I was about to install it my system became unbootable (error screen did not specify why). Swapped drives and now my system runs beautifully, ARK (so far) included.
Not sure if the old drive itself was bad, PS4 system just borked (this time I started with a fresh install) or if the root cause was something else and is still lurking unseen. Will wait and see, I guess.
One other possible factor I didn’t mention before. Around the time the problems started, I had also upgraded my router to a new Archer A7, and I recently noticed that my PS4 download speed was maxing at 67mbps wired, for some strange reason, while my wired PC was maxed at 170mbps. Wireless (5G), though, the PS4 bumps up to 170mbps. Same brand of switch and ethernet for both. Maybe ARK needed more bandwidth? Dunno.