I have been using an Argon One case for my Rpi4 4MB. I boot from an external SSD device (Samsung T5).
I noticed a couple of days ago that Argon had a new version which had space for a M2 SATA drive internally.
Even better you could simply add the new base housing the drive to an original Argon One.
I purchased the base and an EVO 512GB M2 SATA SSD. I booted as normal from the external T5, then used the SD Copy utility to make a clone of the T5 on the internal EVO.
Powered off, disconnected the T5 and rebooted. It was all done in less than 30 minutes and now I have a compact RPi enclosure with an internal SSD.
WO ZS 61
Edgehd 8 with Celestron OAG
Stellarmate on RPi 4
294 mc pro
224 planetary cam
174 guide cam
Sesto Senso focusser
QHY5II-Mono guide camera
EOS6D - not modified