Will I maintain my strength on Foundations

I few years ago I focused all my training using the Building the GB book. Recently I have neglected most of the static holds and want back in. I was able to get one solid one arm pull-up on each arm during this period. I want that kind of strength back.

I currently work 3 Bodyweight strength training sessions each week working on weighted pull-ups (24k for 5x5), push-ups, ring (5x10 rings out at top of each rep) or weighted PB dips (28k for 5x5), band assisted pistols for (5x5) or SL Stepdowns, planks and sprints. I know I need mobility but don't want to sacrifice any existing strength.

Which program would be the best place to start? 

Very strong Justin, I suggest you look at the Stretch course and get that in three times a week with your current training.

https://www.gymnasticbodies.com/gb-courses/stretch-series/

You actual strength should be maintained while a correct progression is the first goal in order to achieve a free of injury body (if you want to reach intermediate level skills). first things first. maintain your strength means that if you can do 1 reps with 30 kg you will make a couple of sets to hold these reps fresh.