I think Slint is quite interesting, but it's licensing model is very similar to Qt, albeit somewhat more restrictive: The open source option is GPL v3 (not LGPL), and then there's commercial licenses, which yes, are cheaper - for now, for a start-up toolkit.

Pricing can change. Yes, there's a perpetual "Buyout" license for $5k, but they can probably just rename the product and claim it's a new one (or add a second product and add commercially relevant eggs to that basket; Qt has tried with things like "Qt for Device Creation" and "Qt for Automotive").

The open source option is not backed by a poison-pill protection like the Free Qt Foundation agreement either.

I know the people behind Slint, and I have a lot of trust toward them. But ownership can change, too.