With the split squats I currently can do sort of the same number on my weaker leg but the execution technique starts to break down noticeably (more problems with my balance for example), so I guess they are not really the same number of reps and I need to be really mindful of the form.

And with the sideplanks I'll need to time myself accurately to gauge. Currently I usually just hold the position as long as I can, often not bothering with a stopwatch, but then it seemed noticeably easier on one side, and I tested on that side whether I could do the progression with the more difficult step of lifting the upper leg and (barely) managed that a few times, but on the other side that kind of extra movement is still impossible for me. So with that it has become clear to me that there really is a weaker sider and I need to have it catch up. And I need to use a timer to not exacerbate the difference.

Generally I'm not so great with using exact timers, because I dislike creating a sort of hectic atmosphere. I tried a couple of exercise timer apps initially, because many people seem to like these for prompts but it drives me crazy to have thirty seconds of this, then of that (and worse with added beeping ::shudder::). So I abandoned that approach. I mean, I realize that with some strength building method timing within and between exercises (and also speed) is important for best results and added intensity or such, but since I mostly want to keep keep my back pain at bay, do only body weight exercises anyway, I didn't bother.

But I guess using a stopwatch for the isometric stuff to exactly time the sides isn't too bad. And I really don't want to worsen imbalances.