Any tips on how to build up strength if you lack the strength to do the easy variations of body weight exercise?

Calisthenics can do wonders, and there are very easy variations of it online. Maybe that is not the answer that you are looking for but I'm overweight and calisthenics changed my life. Try searching hybrid calisthenics online.

I exclusively do body weight exercises. I’m currently starting up a regimen after 5 years of not working out and I feel like I am SO out of shape, even with the easiest workouts to follow. Here’s what I am telling myself: Just do what you can and push through. Even SOME exercise is better than none and the most important part of beginning is to form a new habit. As long as you’re muscles feel fatigued when you’re working out, as long as they’re a bit sore the day or two to follow, you’re doing it. Strength will build from there.

Try doing your training in a pool, like water gymnastics. The water holds up your body and makes it easier to train but you still get The benefits from The work.