I made a gross income of around 3K a month last year out of Royalties on the soft for each device sold. It's Apache 2.0 software so people can do whatever they want.
I started making money when I decided to list on the GitHub README the list of manufacturers/makers that where sponsoring the project. (Only one person at that time) Soon after the others offered to give royalty as well.
I even got a Chinese company, notorious for selling "clone" of OSHW projects, to support the SW development as well via GitHub sponsor.
I've been working on it for the last four years. I entertained the idea to make and sell the hardware myself. But in the end I learned that's it's not something I'm interested to get into. What I really like is working on the software.
It naturally pivoted into a more community driven project where multiple makers are selling various variations of the HW.
I wrote a retrospective last years [2].