For installation to work, I had to use AptioMemoryFix. Also had FakeSMC, NullCPUPowerManagement, Lilu, WhateverGreen kexts as usual.
For audio, I used AppleALC.kext I had from High Sierra installation -attaching it in case someone might need it. If I remember correctly, microphone input wasn't working while on High Sierra but didn't have the chance to test/fix it under Catalina as the BT headphones' microphone already works.
Ethernet works with RealtekRTL8111.kext.
The Bluetooth dongle I'm using works out of the box, as does the Sapphire RX 570 ITX -using Display Port.
I installed Catalina on a new m.2 drive and Migration Assistant was able to copy everything from High Sierra. Even though it gave an error saying it couldn't create my account at the end, it worked.
Sidecar is working but there is some random flickering.
Time Machine wasn't working under High Sierra but works now.
Since it went quite easily and well, I am a bit hesitant to try Open Core now...