Seriously. Please don't settle with bad usability just because you are making FOSS products.
Explore the following aspects of user testing to help yourself out.
“Evaluating and Testing Technical Documents.” Technical Communication, by Mike Markel and Stuart A. Selber, Bedford/St. Martin's, 2018.
Take a look at the propular FOSS products. You will realize that people enjoy using them, and the biggest reasons are likely not that they are free. People only use programs that actually help them. Pricing, freedom and privacy are not as important as usability to the vast majority of netizens. Be realistic.
Let's make FOSS better.