If you don't need the full PCIe slot, there's also smaller SBC boards (either exactly micro-ITX or similar footprint) using the same SoC: The OrangePi 6 Plus and Radxa O6N. The OPi is available readily for $260 with 32GB LPDDR5, though admittedly the RAM is lower spec than what ships on the Orion O6.
These are my favorite sub $500 SBCs available today. Great Linux support and you need to move up to something like the LattePanda Sigma at over $600 to get something outclassing them in a similar form-factor, but then there's no option for directly slotting a full-size GPU either.