After several days of usage, Activity Monitor will usually shows that "WindowServer" is using 6 GB of RAM.
Yeah, 8 GB RAM does not cut it anymore. At least until Apple start fixing the memory leaks in MacOS.
Have you tried turning it off and on again?
Unused RAM is wasted RAM. If your machine isn't reporting memory pressure and/or the user isn't experiencing pageouts, then the machine is well-suited to the user's workload.