$150 PC Engines APU2 (RIP) has 4GB ECC soldered RAM, SATA, mSATA and mPCIE.

$650 QNAP TS435XeU 1U NAS supports 4GB-32GB ECC SODIMM, 4xSATA, 2xNVME, based on Marvell/Armada CN9130 Arm SoC. Debian for Arm64 can be installed via serial console.

Some 4x4 Ryzen Embedded V1000 mini PCs support ECC SODIMM, e.g. https://www.sapphiretech.com/en/commercial/fs-fp5#Specificat..., possibly ASRock.