First thing to decide: fork Leaflet.draw or can I still avoid it somehow? And does it still make sense to stick with the original version?

It's so massively behind, that there probably have to be changes to make it compatible with leaflet 2.x - even if using the full polyfill provided by https://github.com/Falke-Design/Leaflet-V1-polyfill

It doesn't look like there will be any update on this 3rd party lib any time soon. That's unfortunate, but true.
And it's huge (more than 4k lines of code in src).

Also to decide: branch and dependency handling. Should I make a 2.x branch, with a hard version requirement on a (still unpublished) Leaflet integration 2.x version?
There aren't any hard requirements related to PHP. But the PointerEvents switch in the Leaflet library needs consideration.
Would it be possible to do Leaflet.draw modernization in steps? Does that make sense at all?

The only thing that's 100% clear: it'll cause lots of work.