This is exactly the Ardour model (and what Aseprite did before their license change). It's a highly viable way to sustain a niche FOSS desktop app in 2026. Make the repo open source, but put a price tag on the official, signed binaries on the main website. The professionals who need this to get work done will gladly pay a fee to avoid spending an afternoon fighting with Avalonia dependencies and build environments. The starving students and FOSS purists can clone the repo and run the build scripts themselves.
Competition can clone the project in another language and lipstick it to look different.
The real moat is not the source itself but the reputation. Setting the right price point is the actual challenge. You have to find that sweet spot where the price is high enough to fund development but low enough that the friction of paying is less than the friction of cloning the repo and building it yourself. If you price it like a corporate seat then people will fork it out of spite. The goal is to make the official binary the path of least resistance.