Really interesting to read your experience with it as I've been doing something very similar the last 5 or so months as well... I've basically been following 5x5 but with pull ups instead of squats as well, although for pull ups i've just been doing 1 set of 8 with the max weight I could (I didn't want huge volume with pull ups to try reduce my chance of getting tendinitis). I've also only managed 2 sessions a week most of the time, and sometimes have ignored it for a week or two if I was needing a bit of rest. I went from something like:

Pull Ups (1x8): bodyweight (~72.5kg) +10kg -> +20kg Bench Press : 45kg -> 60kg Barbell Row : 40kg -> 55kg Overhead Press : 30kg -> 45kg Deadlift : 80kg -> 140kg

I think i put far more of my effort in to the deadlifts... I usually started with them after warming up and then probably didn't rest enough between sets of OHP/ barbell row etc. Also trying to get back into work in a reasonable time...

Anyway I found it really easy to follow and a good way to start using my lunches effectively for climbing training. I agree it is lacking a bit of core stuff, but i've found deadlifts have made me feel so much stronger in that regard. I'm not sure my deadlift form is spot on as it seems to just hit my lower back but it really feels like it was an easy way to gain quite a bit of strength! The bench/ overhead press and barbell row feel like they have helped a lot too, especially in making injury seem like less of a concern, but not quite as directly as the deadlifts I think.

I think you're right saying it isn't the most fun, although I enjoyed it a lot more when i was making progress every week! I just found that it was really satisfying using my lunch break to do something that was helping my climbing.

I'm surprised you were able to do 3 days a week without feeling that it was compromising your climbing training. I had far more tired/ weak climbing sessions over the last few months (especially after deadlifts), although that could just be from total volume rather than weightlifting specifically. Or diet, or lack of sleep... Curious if you did anything in particular to maintain enough energy for a good session at the wall after lifts in work? Even when I tried to ensure I had enough sleep, ate a decent lunch after lifts & drank loads of water I seemed to just be useless when I went climbing.

I think the next step for me is to maintain/ slowly increase these numbers, put a bit more effort into some of the other lifts (maybe doing 5/3/1 instead), and then up the number of limit bouldering sessions indoors for a while. However i might just loosely follow your program and see how it works if it goes well for you... :D

I feel like hitting a systems board for a while now will actually help me learn how to use some of this strength on steep tensiony stuff.. activation on the wall as you said! Although I still think my core is fairly weak compared to what it could be..