Can't help but wondering, if anyone has looked at or considered the apps Dynalist or Workflowy. Both nicely featured outliners. Not to diminish the work done here, but thought I'd mention it.
I used to use Dynalist to keep all my notes and to-dos for a while; ended up going back to using Markdown files in a cloud folder, with Obsidian on laptop/desktop, and Editorial on mobile (which I purchased years ago). Have been stable with this setup for a while and like it quite a bit; experimenting a bit more with workflows and python in Editorial lately, and also Pythonista.
Anyway, thought I'd at least mention those, in case it's helpful.
There's another decent outliner I tried, but can't remember the name right now. Will try to look it up later.