Together with the Broadcom BCM2711 based Raspberry Pi 4 Compute Module
 this is the only very low cost ARM64 with 8GB RAM I'm aware of.
This said Raspberry is already supported by OpenBSD. RK3399Pro has
the benefit of having four lane PCIe (versus one lane) (and having
USB 3), which makes it way superior for real world IO. Quickly
comparing Geekbench results, RK3399Pro looks about 20% faster than
BCM2711. BCM2711 is uniquely manufactured for Raspberry (sigh), so
there will be no SBCs in the wild with it from other manufacturers.