Hey everyone, I hit this pain point last year and have been working on Keygen, a licensing API for small- to medium-sized apps. I asked a similar question awhile back and was pretty frustrated that I couldn't find a ready-made solution available that wasn't 'enterprisey' (you know what I mean!), resulting in me writing a custom licensing server for the app I was building at the time (before Keygen).
Having to waste time doing that inspired me to create Keygen as a general solution for other apps.
Check it out if you have time. It's currently in beta, but planning to launch within the next few weeks. I have plans for a sub-product to help with Electron app distribution and auto-updates (with license validation).
If you have an open source project or are interested in a discounted monthly subscription price, shoot me a chat request and I'll see what I can do so that you get the most value out of the API.