Hold the volume down button while booting to get to the boot menu. If you get a bootable drive connected, it should show up there. You could connect your windows ssd and it should work fine.

However, the steam deck needs its drivers for the GPU, Bluetooth, and similar. If you plan on swapping back and forth, that may not play nice. Installing portable windows to an SD card really ended up not being that hard and I’d be happy to link you to a guide if you needed one.

If you can find a way to connect an SSD to the deck, I’m not sure if it’s SATA or whatever, if it’s connected and recognized, I don’t see a reason why the deck couldn’t boot from it. Not speaking from personal experience, all hypothetical, I could be wrong. Worth a shot if you understandably don’t want to do a fresh windows install