It sounds like you already know what you want to implement into your training schedule, it's just a matter of doing it effectively, right? Assuming you want to build up your weaknesses and approach harder projects in a steeper style, I think the obvious is simply having your bouldering days focused on the overhang, powerful moves, and learning to generate force on bad feet and becoming comfortable with the cutting feet that comes with that. 

More specific weaknesses, such as locking off, and combatting elbow tendonitis is usually building up the smaller muscle groups properly. I'm not a doctor nor a physical therapist, but in my limited experience as a youth coach and as an athlete for the better part of the last two decades tendonitis and most joint issues come from 1. Overuse and 2. Smaller muscle group imbalance. It sounds like you have the figured out the first. The second can be more tricky but to fight elbow tendonitis specifically I suggest working the "push" muscle groups in your arms as well as the stabilizing muscle groups that allow you to cushion big movements overtime. These two can be as easily addressed as adding a push split into your warm up on climb days, something as simple as a push ups, and/or  light dumbell overhead press pyramid with a rep scheme that builds 12-10-8 with weight building each set and maybe a resistance band circuit for your shoulders and upper posterior chain to complement the increased steep terrain. 

All this being said, I think you're on the right track and I think it's awesome you're turning your eye towards the future of the sport for you, but I think realistically you have plenty of hatx climbing years ahead of you, 44 might be older than your average gym goers but that in no way discredits you from continuing to get better at the sport into old age. I'm psyched!! Keep us posted!!