I’m not sure the RTX series are supported yet. You will need newer drivers then 450xx. But I would have sworn that that the 2000sdGPUs work… and it is the 3000s that have issues.
You should be able to use the nonfree when you first installed Manjaro by selecting nonfree when booting the LiveMedia. Otherwise go to Manjaro Settings Manager in the Menu and click on Hardware and then Auto Install Proprietary Driver and go from there.
I’m not 100% certain but as you have a ryzen nvidia combo I think you need to disable the intergrated graphics from bios. This is a slightly different setup to yours but the thread may help
When I select nonfree in live booting, it won’t boot. It hangs at line like “setting up MHWD”. I waited until I seen [8 minutes / no limit], then I just rebooted with free drivers
I’ve just tried to install https://www.nvidia.com/Download/driverResults.aspx/150803/en-us , it said nouveau should be disabled, I’ve created a file in /etc/modprobe.d to disable it, rebooted, and it doesn’t boot again and even doesn’t show rescue mode…
OMG, disconnection of Ethernet cable solved the problem: https://www.reddit.com/r/ManjaroLinux/comments/iobltn/manjaro_kde_plasma_installation_stuck_at_a_start/