StoreMI is only supported on certain AM4 platform motherboard chipset and processor combinations.
https://www.amd.com/en/products/software/storemi.html
Since AM4 has been superseded by AM5 which does not support StoreMI it is doubtful AMD will continue to publish software updates for an outdated platform.
As to your specific issue, yes the SSD used with StoreMI is for cache only, and it should be possible to boot from the HDD without the SSD copy. Note that if you are using RAID or TPM drive encryption this may prevent StoreMI from functioning correctly. Here is a link to the User Guide.
https://www.amd.com/system/files/documents/storemi-user-guide.pdf
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