Why not split the difference and do both. Hdd for storage and SSD for services/containers. To help with the power load/performance hit I’d recommend using a usb to sata adapter that has external power. Overall though it sounds like a micro optipl x would be great for you. They’re cheap, more powerful, but still sip power, and usually can fit nvme and 2.5" drives together.
Not all ARM chips are in phones, nor are they all locked down like one. There are several ARM devices and SBCs now where switching OSes is as easy as swapping out an SD card. Most do use uboot as a standard and some are even capable of utilizing UEFI.