Despite that, I took a look at the problem (using an NFS-rooted Pi 3B+), and I also found it didn't respond well to having the card removed during writes. I thought it was possibly more reliable using the sdhost (rather than mmc) interface on the SD card, but I managed to crash an ext4 process, after which it was very unhappy.Patient: Doctor, it hurts when I <something random>.
Doctor: Well, don't <something random>.
My old phone did have an external microSD slot. It was very handy for getting photos off it an onto my PC. My current one (yes, it's a bit old), has the SD inside the back cover, but can be removed without removing the battery or SIM card. Just select "Unmount SD card" from the menus and it is fine.