I don’t know about these days, but I remember making a custom layout for Windows back in 2005 that was US Qwerty keyboard plus AltGR+auose for äüö߀ (German umlauts and euro symbol).
I forget how I did it, as I haven’t used Windows for serious work in years.
Obsidian.md has mobile versions, apparently. Whether there is a free sync feature for the ios version will be a Q.