I was in the same situation, and considered https://keygen.sh, but realized implementing one myself is probably faster than trying to integrate a third-party platform.
So, I ended up creating my own system, quite simple, in Node.js + MongoDB, and then I can add whatever integrations I need (currently I only needed Paddle).
So, I ended up creating my own system, quite simple, in Node.js + MongoDB, and then I can add whatever integrations I need (currently I only needed Paddle).