Don't browsers allow you to do form fillable these days? I swear i just filled one with firefox the other day. Maybe that's too limited?
For combining pdfs, pdftk from the command line is my goto. The command line interface for it isn't too complicated.
I switched from ubuntu to debian when 12 was released and it's been fine. Only thing i was worried about was running WoW via lutris but had no issues.
So when my SO windows pc died we bought some newish parts and i installed debian on it as well. Also installed chrome since that's her browser of choice. She's still getting used to gnome, but all she needs is browser, WoW, and libreoffice, which is close enough that it hasnt been an issue. She doesn't even know how to update the system.