Matt,

Addressing the quantity for aerobic base: It’s pretty tricky to quantify what can be lost/regained/maintained.
There’s a more rapid decay of the metabolic factors in a long recovery period, but there are some positive notes- while you will lose your peak fitness, it requires far less volume to maintain a significant percentage of this fitness. Also, if you have a long training history on the aerobic side, you probably have more permanent adaptations that allow to regain peak fitness faster than an individual who doesn’t have a long endurance training history. The answer varies individual to individual, but doing 50% of your peak volume would be great to maintain a base. A conservative strategy to keep the base if you are also adding intensity would be to try to do 80% of your volume below your AeT.

The general advice for most athletes after a major event or climb is to do what makes you happy in the weeks afterward. Make sure that you are addressing the parts of life that may have been strained by training (work, family, friends). It’s great to be active, but specificity does not matter for most athletes. Modulating your training by shifting to another focus is healthy from the psychological side, and likely helps on the physiological side (can’t really tease those apart completely). Most athletes have probably pushed close to or done an overreach during the training cycle, and a big climb adds a lot of stress on top of that (immediate post-event bloodwork resembles that of major trauma for ultrarunners, and I speculate the same is true for expedition mountaineers on tough climbs). It’s important to do what you like to do. If you like to move some heavy weight in the gym, set up a nice gradual return to that and make that your focus.
Coaches across the board often advocate for a delayed return to training, to make sure that athletes have the focus and desire before returning to training. For some athletes the desire and focus are ever present, but for most, a true off-season of cross-training or even reduced activity is completely appropriate.

Matt, Concur with Nate here, I have had a few athletes do quite well with a 50% of peak volume program for maintenance. Get back in the gym if that’s what you enjoy and feel free to explore other somewhat non sport specific aerobic modes like cycling, rowing, running etc while the next goal is not looming super close.

Matt, for maintenance, 3 sessions/wk are totally fine. Especially if those are more fun and motivating.
For long term health and fitness, frequency might be a more important quality – doing more sessions, even if they are shorter, is probably more likely to promote long term adaptations. Also, 1hr should be a very manageable load, which is an advantage if you are trying to have a consistent year-round program, leaving you plenty of bandwidth for everything else in your personal or training life.