I seem to have got things working on bullseye. I'm not sure how many of the following steps are essential, except the last one!
Also, though this may solve my immediate problem, the issue that qtcreator doesn't work "out of the box" remains. If something about the interrelations between packages makes it impossible to configure automatically, there should at least be guidance about what to do.
1. Install qtbase5-dev.
2. In QtCreator Tools | Options | Kits select the Qt tab. Apparently the link facility only works for stuff installed
in a certain way, not including distribution packages (or so says the internet) . Use add to point to the qmake which is now
available because of 1.
3. Also needed to set Qt version in the Desktop kit, which took a couple of tries (maybe need to restart to have it register).
4. Install qtquickcontrols2-5-dev when QtCreator still says"Unknown
modules in Qt: Quick". This is the only qtquick.*dev package, though
it hardly seems on point. Didn't help.
5. Install qtdeclarative5-dev. Project now builds.
My guess is that all but #4 are necessary. #5 is specific to Qt Quick; the others are not.