Custom Templates is a feature I go back and forth on, mainly because it'd complicate the data model a lot, but also some parts of the product. I might get around to it, but I'm going to work on other things (Android version, Apple Watch companion) first.

Does the data model have to be changed? Can't it just be a simple "fill exercise and how many reps/kg you want" type of thing?

And understandable with your limited time!