I went with option 3 using Paddle for my solo app (https://checkbot.io) and don't regret it even though it didn't seem nearly as popular as Stripe. I receive a single invoice a month that I declare as income on tax returns and that's it - I can sell worldwide and don't have to worry about county specific tax issues. Gumroad and Fastspring are similar I think but I haven't used them.

Option 1 sounds like asking for tax trouble to me and especially if you're small option 2 sounds like a lot of work where you're likely to make mistakes if you don't have a good idea of what you're doing and have the bandwidth to keep up with changing regulations. Selling for a year then finding out you've done the taxes and VAT collection wrong sounds like a world of pain to fix to me.

Willing to be proved wrong but I'm glad I ignored suggestions at the time that I could have saved a few % per transaction doing it myself. Getting good tax advice within my own country has been difficult enough so the merchant on record approach feels easily worth the cost for me.