You have to apply the Post Install Volume Patches (selection n. 3 in OCLP TUI). In addition you didn't detailed your system: is RAM extended (>4Gb), is your internal disk an HD or SSD (SSD is the best) ? I'm writing from a MBP like yours that is very performant and without any artifact and glitch.I installed this on my 2011 MBP (15") with broken AMD Radeon GPU. This was after I installed Catalina using this: Catalina on 2011 MBP
However, sleep and screen brightness control did not work. Overall the speed was VERY slow. Loading app was slow, watching video online was very jerky in the beginning.
I ended up reinstalling Catalina.