Update: I’m going for it. The only games I have on the SD card I don’t care much about. The other games are on the internal storage. So if this doesn’t work out, I’ll just reformat the card.

I ended up using KDE Partition Manager since it’s included in the Steam Deck Desktop environment.

This is taking a long time. A quick web search says that’s normal. Probably would have been better off just wiping, installing Windows-To-Go, resizing the Windows partition, and then adding a Linux partition.

It’s been over 20 minutes that’s it’s sitting at:
Move the file system on partition 'dev/mmeblkop1 to sector 999,874,560