The SSD is removable and upgradeable, there are 3rd-parties managed to produce a compatible SSD card that fits into the proprietary slot. With the same or compatible NAND chips on the replacement SSD card, upgrading the SSD on the Mac mini M4 turns out to be quite feasible.
"Apple's unique SSDs aren't sold by Apple and aren't currently available by a third party." is no longer correct. https://expandmacmini.com has tested options to upgrade SSD to 2TB.