I’ve used Xournal++ and Write, both worked pretty well. Saber also looks promising.
It’s going to come down to how the program handles smoothing of the pen input, which is going to differ based on how noisy your tablets data is, and on your handwriting.
Yeah, most dead scripts have Unicode, specifically because how the hell would you write academic papers about them in this day and age otherwise? Even old Irish Ogham:
ᚅᚖᚙᚗ
The line is a convention, because ogham was originally written on the corner of a stone stela.