"I can buy a WT ticket, and some of the fare classes I can get will be upgradeable with miles to WT+"

Or to Club World where WT+ is not offered. You just need to phone and ask for the a fare in Y,B or H class ( H is the cheapest )

"Then, if I buy a WT+ ticket, every fare class I can find will be u/g'able, right?"

Indeed. Revenue WT+ is only booked in W and T classes and both of these are upgradeable with miles.

"Same with J?"

Indeed. Business is booked in J,D or I and are all upgradeable.

Dave