That’s interesting that an external drive is faster than the internal ssd! That’s something to think about. I have a 1TB usb-c drive I use for photos and such, but my main problem with the internal 256 is iPhone backups won’t fit very soon, and Apple doesnt let you specify an external drive to backup to. I feel better using the internal drive as a boot drive, but a true thunderbolt enclosure might be better?I have a 2020 27" iMac with the internal 256gb Apple SSD (blade, permanent). I picked up an external OWC Thunderbolt 3 enclosure with a 1 Tb Samsung 970 EVO+ nvme SSD and couldn't be happier. The Thunderbolt 3 enclosure/ssd boots faster than the Apple 256gb blade.
I never reboot unless I need to for updates, so boot times I don’t care to much about, but this machine is not slow to boot by any means.