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The Compute Modules have (optionally I think) eMMC which is not especially fast, but fast enough and appears much more reliable than SD cards.

The RPi 4 also has support for USB 3 disks using UAS. I've attached a couple of SSDs to mine. https://rwmj.wordpress.com/2020/09/24/raspberry-pi-4-running...

I think recent firmwares would allow Sd-card-less pxe boot. I was wondering the other day if you could then run the whole system from RAM disk only and/or maybe mount volumes via iSCSI from SAN (keeping in mind limitations of RPi 1Gb nic).

Edit: someone has done this already. Of course! :) ( https://tech.xlab.si/blog/pxe-boot-raspberry-pi-iscsi/ )

Could also use the sd card to boot only, then pass everything to a USB3 SSD.

or just boot usb :)

Yes, with the compute module. https://pipci.jeffgeerling.com/