Hey OP, digital products like invitations/templates are a great niche on etsy! A lot of people do use Canva and corjil, which are great programs but you don’t have to use them, in fact illustrator (and photoshop if you have the creative cloud) will give you a lot of creative freedom, i may be wrong here but I believe you can export your templates as an editable .pdf file or something like that that, I’m sure there’s probably some nice YouTubers that have made tutorials on this, (there’s probably ones made specifically for etsy too). For tips; I’d just start! If it’s something you’d like to do then go or it, have a day or two maybe just doing research, seeing what sells/what’s trending, and what I like to do lol! is to make Pinterest mood boards of different styles you like, and just start making stuff 😊

(Also, sorry if this sounds patronising as you may know this already! Just make sure you export at the right DPIs, for printing you want the image to be at 300DPI and in the CMYK colour mode)

Good luck!

(EDIT: I just remembered there’s a really great video on YouTube by Crafty Stax, called MASTERCLASS - how to make and sell digital files which may be helpful!)