Hi Ahmad!
I use personally nfty and I am happy to provide reviews to MRs on Salsa for the various subtasks you are working on here.
Simon already replied on your questions. Additionally I suggest you ask upstream what they recommend as the best was to build ntfy without Stripe. That way you can maybe have a build tag or flag that will be aligned with upstream and you don't have to maintain the delta forever, and future version update imports into Debian will be easier.
Otto