I see that you said that you have your own training program. What are you looking for the app to do for you? I have my own training program too. What I do is I enter that into Garmin workouts as a customer workout for each day of the week. I use that then to help me track what set I'm on, be consistent on rest periods and as a sort of backup on rep counts. There are limitations to the rep counting accuracy as you would expect with any device that uses an accelerometer but I have found it good enough for the most part. I can then later review the reps, weight and sets on Garmin Connect if I want but I keep a paper log of all my workouts anyway so it's more just to keep me on track while I'm working out. If that's what you are looking to do, you don't need any app. Just create the workouts in Garmin Connect, download them to your watch and go from there.
If you use iOS try Strong. It's really good (especially the paid version) and developer has been active.
I'm not sure but if you use your watch during the resistance training it _might_ consider them as an endurance/interval training. Also, the Garmin workout app does not allow to do any other exercise except the ones on the list...
