After trying out several snapshot builds and a plethora of settings regarding my ethernet adapter I switched out the Cat6 flat cable to a new Cat7 one. I had some slight suspicion after seeing that a fixed 1GBit link rate would cause very frequent port flapping. I'm at 7 days uptime now without having a single issue.
It never occured to be that something as trivial as the cable could be the cause of issues in my case. Especially since that very same cable worked flawlessly for years with my previous router (TL-WDR3600). Would appear my router model is more sensitive towards cable quality and shielding.
I'll keep an eye on the logs regardless, but at least for me the problem wasn't induced by the mt7530 bridge nor OpenWRT.