Out of curiosity what types of things do you guys store in the SD cards? My 256GB SSD drive is around 70% full but I don't like storing things on an SD card because the read and write speeds are slow.
My Surface Book at 256GB is fine for me, for now.
My Surface Pro is a 128GB. I have a 128GB card that stays in it, that I keep my media on. Pictures, movies, music. Basically, things that are generally read only. As you note, the write speeds aren't that great, and they don't need to be read fast.
You do want to keep as much free space on the SSD as you can. If you run it low on free space, it will read and write and rewrite the same blocks constantly for temporary files, and that shortens their life.