In reply to TonyB:
> Recently, I've been doing quite a lot of circuits and am probably the fittest I have ever been but am not certainly as strong as I used to.
Yep - that makes sense. When you build aero-power, your anaerobic capacity goes down. Striking a balance between being fit and being strong is a fine line and needs to be considered in context of your target routes. No point in being overly strong on a stamina route! COnversely, being super-fit, but not being able to pull off the crux move is equally unproductive.
> I'm pretty keen to switch focus to getting stronger
Ask yourself why? Are you actually weak, or is it just your ego saying you want to be stronger?
> Ideally I think the best way would to identify 5-10 key exercises that I can use as reporters of strength/endurance levels.
1, % body weight your can hang on a single joint edge (one handed - change % loading with additional weights (you're too strong already) or decrease with a pulley & weights)
2, Max time/number of moves foot-on campusing would be a good test of max aero-power. Dave Binney has correlated #moves with angle of board and max on-sight grade. Tom Randall has an even better measurement on his "lattice board" test, e.g. 70 moves = OS'ing a resistance based f8a
3, The hard bit is getting all your energy systems working in a balanced way that is specific to your aims. Measuring this isn't easy and describing it in a single forum post harder still. If you're serious about doing it right, as someone else has rightly pointed out - don't ask here, get the input of a proper coach who knows their stuff. (Ask Tom - he's a co-owner of your local wall!)