Even you like open source, QucsStudio isn’t open source. Going closed source may be good for ads or sponsorship logos in the software, so no one can fork it and remove this content.
I have an idea: Qucs-S is in active development. It uses C++17 and Qt6, runs on Linux, has more part libraries and more simulations. I think QucsStudio should be more like an addon/plugin for Qucs-S. Currently Qucs-S does not have an integrated addon store, but this would be nice. Maybe there is a way, QucsStudio can use Qucs-S as some kind of library.
The current situation is a dead end of QucsStudio. Why should someone build on something, which is maintained by only one person in spare time and closed source? A product that could be abandoned every moment? Why should someone crowd-fund or donate? – A closed source payed software from a big company with 1000 developpers or an open source software with a big community is much more stable.
I talked to ra3xdh. He said, he is willing to merge QucsStudio and Qucs-S, if possible. That would drastically reduce the amount of work to do for future development. But that means, QucsStudio will be open source again. Paying for features and support will stay possible and may be easier to acquire.
To protect the intellectual property in some way, a trademark can be helpful. Everyone who creates a fork, can’t use the trademark without permission. The sponsors may only rely on the original one. Also the forum with its content can’t be forked. The sponsors can place there logos there. And the forum may be more successful, if there is only one Qucs with both feature sets (Qucs-S and QucsStudio).
Maybe a cloud or web option for collaboration may be interesting. This may also be useful for teaching. That means, monthly income can be generated for cloud access.
Maybe research centers and universities may pay for support. For in person workshops, makerspaces may be a good place. They can organize everything and pay you only for the workshop.