So I managed to get up-to-date and running nvidia-390 with Plasma 6 [and even Wayland (but only @480p)] with kernels 61, 65, 66, 69.
The last and only iso I could do it with was manjaro-kde-23.0.4-231015-linux65.iso
The ones before it might be doable, but you run into dependency-cycle issues with plasma5>plasma6 packages on pamac/pacman.
The ones after, the isos came with linux66 or newer and proprietary installs (my graphics card is NVIDIA GF119M [GeForce 410M]) would not boot past the logo on the 66+ kernels.
As this might be scaring off “new users on older hardware” I believe this is useful info for devs here.
There must have been some changes in either Calamares, or possibly Mhwd from the 23.0.4 to the 23.1.0 release! Sddm I don’t think is an issue as I selected auto-login on boot in Calamares.
As this is not only concerning this Wynsdey release I can provide info in the thread I opened here for the issue: Nvidia 390 not working with kernel 66 on new install?
Btw, where can I find the plasma-to-defaults.sh script kindly provided by @linux-aarhus ?