1. there's NONE Linux distro out there that is simply Qt based; what you're probably asking is about the Desktop Environment one distro or another promotes as the default one on install. openSUSE let's you choose KDE as the default DE and in fact has been its historically top option (it used to be Gnome back in the Novell days, but is not (en)forced to new users anymore); of course KDE is based on Qt or better said, it 's "The" one genetically tied to Qt ever since birth and cradle.
Note that you're not here being obliged to KDE whatsoever, although it is the only and the one I use on Linux for two decades now ;)
2. Calf plugins are really going under the heavy pressure all these days; just look at that Ardour spell a course on them.
Most probably, all the CALF issues on Ubuntu Studio that you may experience have nothing to do with or without it being a Qt/DE based distro. It is rather an intricate software level misalignment in your system. There are (more than one) reports that things get solved or work much better with the latest Calf git head, while other's get it all right only after they build (compile) parts or all of the software on their own premises.
AFAICT
1. there's NONE Linux distro out there that is simply Qt based; what you're probably asking is about the Desktop Environment one distro or another promotes as the default one on install. openSUSE let's you choose KDE as the default DE and in fact has been its historically top option (it used to be Gnome back in the Novell days, but is not (en)forced to new users anymore); of course KDE is based on Qt or better said, it 's "The" one genetically tied to Qt ever since birth and cradle.
Note that you're not here being obliged to KDE whatsoever, although it is the only and the one I use on Linux for two decades now ;)
2. Calf plugins are really going under the heavy pressure all these days; just look at that Ardour spell a course on them.
Most probably, all the CALF issues on Ubuntu Studio that you may experience have nothing to do with or without it being a Qt/DE based distro. It is rather an intricate software level misalignment in your system. There are (more than one) reports that things get solved or work much better with the latest Calf git head, while other's get it all right only after they build (compile) parts or all of the software on their own premises.
As always, YMMV :)
Cheers