I'm glad it worked @JDW! I think you're right that the ROM patches Turbo040 applies at boot is the culprit. ROM disk support like in the base BMOW ROM image also don't work with Turbo040.
Have you seen that forum member @ZaneKaminski is creating a new ROM SIMM with many of the same features as ROM-inator? Maybe Zane's new ROM code can fix HD20 support with Turbo040 sometime in the future.
The SD2SCSI (v5.1) works in all my other classic Macs as it is, without any configuration changes and with the same SD card and filesystem, so it's unlikely to be an incompatibility.
I've run out of Mac compatible SCSI drives now as they all seem to have died with age.