Your config must definitely be different than mine.
In PhotoLab (versions 5 and 6 tested), I get the launch panel - but only for the Nik v6…
…and the Nik v6 app launched in my test with “Nik 6 Color Efex”. So far so good.
In Lightroom Classic (current version), plugins and templates are installed side by side…
…and I can select to use either version…
…even though “Nik 6 Vivez” has lost its trailing “a”
@StevenL: Please have Lr modules checked for the “vivez(a)” typo.
Addendum: I’ve cleaned out all things Nik before installing versions 5 and 6 for this test.
As the Nik uninstaller leaves a lot of things behind, I use “Find Any File”, search the drive a few times and trash everything Nik, except the license file. PM me if you want to know how to completely remove Nik stuff.