Presumably the qmake plugin will eventually be ported to core20, but for now it is probably easier to stick with core18 in the interim.
The compiler in Ubuntu 18.04 should have pretty good C++17 support. It sounds like the problem instead is the Qt version based on this Stack Overflow answer:
While you can’t use CONFIG = c++17, it sounds like the older Qt will accept CONFIG += c++1z (the alias the standard was going by when they didn’t know when it would release). If that doesn’t work, QMAKE_CXXFLAGS += -std=c++17 should.
For the record, CONFIG += c++1z proved to be sufficient, and judging by package names alone it appears that GCC 7 is what ends up being used, which is indeed almost fully C++17 conformant — only some parts of template argument deduction for class templates are unimplemented in GCC 7 per gcc.gnu.org.