It’s been a while since I set mine up, but I recall the boot partition had to stay on the SD card for some reason. I forget the explanation, but something about the way the Olimex box is configured depends on using the SD card to boot.
The way I got my system onto an SSD was to flash the SD card with the normal image, then migrate the Btrfs partition onto the SSD using btrfs replace following this old but still relevant thread: support to move system to SSD/HD (btrfs replace / ? + cp -ax) (#435) · Issues · FreedomBox / FreedomBox · GitLab
You can do btrfs replace while the system is on and running. I recall it didn’t take too long either. Afterward, your system will all be on the SSD except for the boot-related stuff. The SD card more or less just sits there doing nothing, unless there is a reboot and then it will answer the call.