Had you been looking for "in-band ECC", the cheap ODROID H4 PLUS ($150) or the cheaper ODROID H4 ($110) would have been fine, or for something more expensive some of the variants of Asus NUC 13 Rugged support in-band ECC.

For out-of-band ECC, e.g. with standard ECC SODIMMs, all the embedded SBCs that I have seen used only CPUs that are very obsolete nowadays, i.e. ancient versions of Intel Xeon or old AMD industrial Ryzen CPUs (AMD's series of industrial Ryzen CPUs are typically at least one or two generations behind their laptop/desktop CPUs).

Moreover all such industrial SBCs with ECC SODIMMs were rather large, i.e. either in the 3.5" form factor or in the NanoITX form factor (120 mm x 120 mm), and it might have been necessary to replace their original coolers with bigger heatsinks for fanless operation.

In-band ECC causes a significant decrease of the performance, but for most applications of such mini-PCs the performance is completely acceptable.