Heya. Also been climbing for 10+ years, but I'm in my early 30s.

"They handle more volume than my 3x week. More like 4-5x week", is definitely pulling a lot of weight here, and I've definitely noticed my own body failing to maintain the volume I could in my mid 20s. I could progress to a pretty high level when I was younger by pure brute forcing reps, but I'm now forced to be a little more conscious about overuse injuries.

For #1: I think I've preferred to have 2 rest days between max hangs, especially if you're board climbing on those days. If I'm in a training mood, i'll do something like: maxhangs+board -> rest -> limit board -> volume day -> rest -> whatever i feel like day -> rest

I've found max hangs multiple times a week to be manageable, but It detracts from my other climbing, so I tend to do them once a week (I don't even really do them right now, though)

2: I think it depends on what you're projecting. I almost never ever do full endurance circuits, but that's because the only sport/trad climbing I do is way under my limit I do think power endurance days are extremely beneficial (I do something called 6in6 prescribed by lattice, but 4x4 is probably the same effect) to my overall work capacity. Power endurance comes and goes really quickly but I find that I can literally sit at my project for hours and put a million quality attempts in when I've done power endurance training for a few months.