I think I've thought of a better way to progress to Handstand Pushups. Go through the usual steps pike pushups, the L shaped ones etc. Once you build up the reps and are looking for the next step.

Do a stomach to the wall handstand but out a little bit more than a normal one. Also let your hands be as wide as need be to push with max strength. From here your chest can help a bit more.

To progress either focus on bringing your hands in or moving closer to the wall. Fix one of them at a time. I've been doing it for 3 weeks and already notice a big jump in my HePU pressing ability. I went from 2 reps as a max to 5

Once you're doing HePUs for the progression standard. Just add a little book on each side.