Personal Trainer/Runners coach - auto adjusting workout planner

Is it worth putting any effort into this before HRM is better?

Well, if you have a bluetooth ECG it can effectively replace the Bangle's built-in HRM (at least as far as your app is concerned). So you can always develop with that if you have problems with the Bangle's HRM and then later when it gets better you can use that.

... but generally all light-based heart rate monitors do struggle if you're moving your arm around a lot - so if you're doing serious exercise an ECG is probably the way to go anyway, regardless of if you're using a Bangle or not

Should all this live in the bangle app, or some of it live in e.g. Gadgetbridge?

Personally, I think if you can do it all in the Bangle, that'd be better. It'd then work for iOS users - and sometimes it's nice not to have to take your phone with you.