I don’t climb - but for me it was doing weights I had to plan for return (upper body)
I did a lot of reading around on line - I found a blog from a weight lifting lady who had had a mastectomy that was very useful about planning a return to weights, and cancerfitme was motivational, but lacking in detail (although, as they point out, that would be impossible to advise on)
So - I got going on walking/ jogging again (double bra-ed), and carefully listening to my body (not something I normally do - I usually power on through!), and started lower body/ glute/ core work. As my wounds healed I then have slowly started reintroducing upper body weights - but oh so carefully (and I have learnt to stop at the slightest twinge); and I did find that some weights I thought would be ok, weren’t (more to do with the fascia they cut in the op, rather than the muscles I think). Unsurprisingly, lat flys are still problematic.
Frustratingly, radiotherapy has knocked me back (both for muscle strength and inability to do some exercises with pain, but also fatigue), but back to getting going again now!
I did the stretching exercises post op religiously x3/day which must have helped as well btw.
Carrying a backpack has proved an issue - and caught me out during/ after radio. So I am avoiding for now - more discomfort. I worry about lymphoedema also - apparently walking poles will help when I get to that point, and I am looking for a compression arm sleeve too (I know they are around - runners andcyclists use them)
Hope this is helpful - enjoy your return to the mountains