iMac 8,1, HDD, installed Monterey, not a good experience, fallback to what?

Today was my first experience with Legacy patcher and stuff.

My plan was to install a newer OS to run an specific mac-only software. The current version of the software requires 10.11 (el captain), which is the latest version supported by the iMac 8,1 i'm using (Core2Duo, 4gb DDR2, 128mb vram). The odd situation was that some features of the software weren't running properly on 10.11, so I decided to go a little newer and be a little more future proof, and install Monterey.

Also i'm like testing the waters at the moment. Eventually i'll open the imac and install a Sata SSD, for the final build, which I intend to use to work.

Anyway, I'm getting some video glitches on Monterey (maybe the lack of metal?) and even sometimes crashes or freezes the entire system. Sad. I disabled the transparency on the system and it got better, but I've noticed that some apps trigger the glitches (firefox, Steam, Safari when using background blur)
The software I wanted to use to begin with, is running but a little slower than i expected.

So, some questions:
1. which would be a good version to downgrade to?

1. how to downgrade? Just run the Open core legacy patcher, and created another installer, and install over the current system? (for the final final build, i'll do a clean install)

2. Once i change settings on the OCLP 1.5.0, do i need to do anything else to apply the changes? or a reboot is enough?

Bonus: Some nice screenshots of the video glitches I got. I'll take more and likely use as wallpapers. They look cool.

Thanks!