You are fine with a free software project using Slint as well: Slint is a GPL project, with everything that implies. The releases are out there and the slint project is bound to the terms it released them under. In theory we could release new versions without the GPL option, but we can not take the sources of the released versions away. Neither the other licensing options nor the contribution rules change that. If youbare happy with GPL dependencies, you can use Slint just as any other GPL dependency, with the same risks and benefits.
The copyright holders of any GPL project can decide to relicense their (future) releases. I admit that it is a bit simpler in Slints case due to the contribution rules, but other projects have similar rules in place. Copyright assignments, CLAs, ..., they all exist to simplify a possible future relicensing effort. And even GPL projects without such provisions in place have manged to relicense before.
As a user of Slint you typically never get into contact with our contribution setup at all. Only a contributor might pause and decide not to spend time on Slint due to that. IMHO that is entirely normal: I use tons of free and open source projects that I would never contribute to -- for various reasons ranging from contribution terms, to programming language being used or the projects community.
In many cases you can also publish slint related code in your own repository under whatever license you like. While this obviously does not work for core functionality that has to live in Slint itself, it does work for a wide range of things you might want to make available (like new widgets, ...).