Hello,
From some months now, I'm working on a minimalist solution for my own invoicing needs. I thought it was better to push this forward, and to finish the job: I've made https://fossbill.org
Fossbill is a simple billing tool. It allows to manage customers, products, and bills. We values efficiency, reliability, and accessibility.
I also made a documentation companion website available here: https://docs.fossbill.org
The project is open-source, is licensed with AGPL, and is really easy to self-host. My instance is a SaaS that cost 10 euros monthly.
It is the Steam Deck without the screen. Just the controller I ever wanted