Why mount the SD card read/write. Why not mount read-only. After boot, I can remove the SD card. I can insert another card with more files, or chroot to other external storage. The rootfs is in RAM. Been booting RPi with tootfs in RAM since 2013.

SD cards are not reliable for frequent, small writes, IME. Certainly worse than USB sticks. I wish RPi offered the option to boot from USB. Even an option to use a SD card to eMMC adapter and boot from eMMC would be an improvement.