Hi everyone!
I have same problem here.
I upgraded from PT12.7.1 to Ultimate and started to have this issue.
I´ve tried every way you guys mentioned but problem persist.
First I Installed PT Ultimate 2020.12 on my OSX10.13.6 opened a mix session including video (dnx-hd).
System usage shows 100% RAM.
Same session running PT12.7.1 on OSX10.12.6
64% RAM usage.
Later i´ve install PT Ultimate 2019.6 and RAM usage went down to 95% not showing warning but still very tight.
I gess it is a cuestion of finding right convination of PT version and OS.
I´ll try with an earlier version of PT (2018.7) today to see if i can reach the 64% i had with PT12.7.1.
Anyway, any clue is very wellcome.