Ok, so my fuel gauge will quickly and smoothly go all the way to full when I turn the key to the "on" position, stay there when I start and drive the car, and go back down as it normally does when I turn to "Acc" or "off".

This started when I last filled the tank two days ago, should be near the halfway point right now.

I've disconnected the battery to try to reset things, that hasn't helped. Not seeing an obviously related fuse I could pull, at least not up front(not sure if there's anything closer to the sending unit to check).

In recent weeks it's had intermittent problems where it wouldn't go all the way up when filling, like it would go to half, stay there for 30 miles, then the next day go nearly to full(minus what I'd expect for 30 miles travel) when start her up to go into work. Or it would rapidly go to 1/4 and then slowly climb to full as I drive. Unsure if this is related, but it's the same system so I figured it's worth mentioning.

Fuel pump assembly, including the sending unit, was replaced about a year and 20k miles ago. I haven't had it run out in that time, though the idiot light has come on a few times.

Possible sign of an instrument cluster problem would be the odometer display flaking out and needing to be cycled through trip to actually show my mileage, and Torque reporting slightly higher RPM than the tachometer does. Obviously these aren't fuel issues, but might point to a gauge issue with a good sending unit maybe?

CEL is on, with P0442 Small Evap Leak which has been there for months so I doubt it's related to this problem which just showed up.

Anything I can try for diagnostics/repair that doesn't involve dropping the tank?

No drivability issues, it's just the fuel gauge is not exactly useful right now.