You can make use of Figma integration for something similar to Qt Design Studio.
Too many folks nowadays don't seem to fully understand how powerful GUI designers for native code used to be.
You always get some arguments about pixel perfect positioning, completly ignoring the fact most of them had layout managers available, even VB pre-VB.NET (yes Windows Forms does support layout managers).
Don't want to pay upstream? Also don't get to charge money.
Want to pay up stream? Another license can be arranged where both parties get to earn money.
The reason why it's not possible to include GPLv3-only code in a GPLv3-or-later codebase is that the latter is more permissive, allowing the FSF to release an updated version of the GPL.
They won't make GPLv4 any less copyleft and more permissive than GPLv3, if they ever do make one. At worst, the GPLv4 will cause some commercial user of the code to be even more inconvenienced.
But to be fair to Slint, they do have a free proprietary license which can be used for developing proprietary applications as long as it’s not considers in the “embedded” space.
I am not asking them to remove GPLv3-only and replace it with MIT, I am asking them to use GPLv3-or-later.