Warranty it. Even though it’s out of warranty. Fisher will pretty much replace any Reverb by default. Go through an LBS but got to find one that will send it off for checks rather than just charging you for a service.
As for others… while Reverb gets a lot of notice about issues, it’s purely because they massively outweigh the competition. I’ve yet to hear of a reliable dropper. Besides that, Reverbs can be had pretty cheap and warranty/support is excellent.
p.s. I’ve found cold temperatures do affect the return speed, and the slight sag is common though first off just check the pressure is bang on 250psi. Noting how many shock pumps are inaccurate, especially up at that end of the scale.