I have a Mi 5 Pro (4GB RAM) and have 1.9-2.4GB out of 4GB available normally, and this has been like that since forever...
If you got the 3GB version, then 70-75% of used memory makes sense.
Today install 8.11.22 version and ram use better
I think the problem is in version 9.3.21. Yesterday i had tested with clean installation and first setup 80 degree very heating.