I routinely use about 100–150 Unicode symbols (in addition to ASCII), mostly Greek letters and math operators. I find it much simpler to remember the \ sequence since I already use it for LaTeX, and just set up Emacs to complete/replace these. I don’t think I would be touch-typing if I had to remember where eg \varphi is — instead, I would spend 5 minutes finding it on the keyboard.
I even replaced my Android launcher with
https://github.com/fAndreuzzi/TUI-ConsoleLauncher
so I can just type app names.