Ok, after 2 days I can confirm 100% that my problem was simply the kernel.
Here’s what I did: 2 days ago I reinstalled the latest Manjaro ISO with Gnome, keeping all the default bios settings except “Power supply idle” set to “typical”. So, no ram settings change and C-state active.
With the kernel 5,7 (default in this iso) I immediately had problems with the wifi card (connection that comes and goes continuously) and several freezes. I therefore installed the 5.4 LTS kernel, choosing from grub to use that. And there you have it, no freeze in 2 days and perfectly stable wifi connection. I tried again to revert to kernel 5.7 and the system freeze in 3 minutes.
So now there are no more doubts, in my case the only problem was the 5.7 kernel, perhaps due to some incompatibility with my wifi card or with this processor in particular. Anyway, my problem is solved for now.
Thanks a lot to anyone who responded and helped me to solve the problem.
