Greatly appreciate the thoughts ziambe, thanks. It turns out going from -35 to 75 LR did the trick, and when testing him out this morning decked out in some LR charms, he was quite safe, even with the indestructible 3x Sol shield which means I can fully leave him to his own devices. I ended up finishing my first clear of the TC90 monsters (thanks ziambe for the excellent spreadsheet summaries earlier in the thread), and while I didn't find much (a Shako dropped in the last 5 minutes in WSK), it was a good proof of concept that I can actually do runs. Big thanks to froz as well for gifting me the starter gear for actually killing in 1.08, I don't have all that many sets or uniques in 1.07 so this helped a ton.
The next step I imagine will be to equip a sorc to tele around and wake up the monsters. Also need to forward some mules that are allowed in Act 5 Hell, I think I have enough of them ready to go, hopefully. Also figure out how to actually equip the Barb so he can do the runs quickly, I imagine a lot more FRW might be smart as this first run took forever. And +Max damage charms maybe, the kill speed wasn't the best either. But still, a decent start!
ETA: I set up for a second run today, and had forwarded a few characters so I was playing players 4. They brought some +20 Max GCs and a bugged LL craft with them from 1.07, and the ring was a great help. Still, even p4 felt like a slog to get through with my starter gear, so I will either have to come up with better equipment (probably hard), or start pre-teleing with a sorc soon.
Anyway, I accidentally saved and exited out of the run in the Icy Cellar. Awkward. Got my first TC90 in the Frozen River though, didn't take a screenshot because I didn't realize it was a TC90 until froz told me on discord and I went back to get it:
Oh well, at least this first day proved things are working.