the SW30*DA use coreboot bios, probably not secure enough for you, but at least open.
they're pretty cheap, since they're old: https://www.ebay.com/itm/235615584195
there's a long thread (not bios centered, of course) about these (and other cheap x64s) devices at Tips for getting cheap used x86-based firewall with full Gbit NAT (a PC Engines APU) if you are in the US
the RPi5 is supported by openwrt.