Thank you for the hints, guys. There`s been much change in the technology in the last years I wasn't aware, and I made a wrong purchase which doesn't suite my needs.
I have decided on short term to:
sell A320M-A PRO Max motherboard
re-use the CPU, GPU, power supply (current 600W), HDD, memory
buy another second hand motherboard which supports the existing CPU - Asus Prime B450 Plus AM4 ~65 euro
6 SATA ports
SATA 5/6 are shared with M.2 device, im not yet sure if I would be using that, the format is new for me.
1 additional PCIe lane x16
buy the cheapest enclosure I can find Gamemax Fortress TG -> ~40 euro
buy a PCIe expander SAS/SATA to get 4 more additional HDD slots
If my understanding is correct that would get me 6+4 => 10 HDDs.
The 10HDDs as storage + the GTX 1660 SUPER hopefully will be sufficient to stream 4k on Plex + transcode for Frigate cameras, reaching my end-goal.
I kindly ask for a feedback from the community before I made any changes, thank you!