Is a downgrade from Monterey impossible on my MacBookPro from 2009?

Hi, I upgraded from El Capitan to Monterey absolutely flawless on my macbookpro9,2 with the help of OCLP.
Because I find Monterey somewhat laggy I want to downgrade to earlier versions like High Sierra or Big Sur to test if they run better on this Mac.
So I made an new installer with OCLP 1.3.0 in Monterey. The installer medium boots fine but when I try to pick the internal drive in the Installer I cannot install. If the drive is formatted with APFS the error says the drive needs a firmware update and I should try a drive formatted with MacOS Extended (journaled) instead. If I format the drive accordingly the error says now" This Update cannot be installed on this computer".
I'm desperate. help would be really appreciated