Found the replyd I am now powering all 5 drives (and I also tried 6 drives) from the two 4 pin ATX ports labeled SATA_PWR1 and 2. The pinout can be found in the Topton N305 listings at AliExpress.
No extra PSU needed, all from one brick. Seems to be running solid, even with all drives in full motion and about 100% CPU.