I happily booted and ran an Intel Mac mini using a 4TB drive in a Thunderbolt 3 enclosure, and I do the same for an M4 Max Mac Studio using an 8TB drive in a USB4v2 enclosure (OWC Express 1M2 80G).

You'll just have to be careful to match the enclosure to the ports on the system. The base-model M6 Mac mini still uses Thunderbolt 4, so a USB4v2 enclosure would be wasted.

Funnily enough I do the same with my Intel iMac... I have a 2TB Thunderbolt 3 Glyph drive as my primary and boot drive and it works a dream... if I got one of these new Mac minis without upgrading the drive, presumably I could just plug this straight into it and apple will do its magical migration thing?

These Macs are reportedly shipping with macOS 27 Golden Gate. You'd have two incompatible constraints: Macs don't like to run a version older than what they shipped with, and Golden Gate doesn't support the Intel iMac.

You might be able to boot from the internal drive, install/upgrade the external drive, then boot from it.

Sure, yes, compared to the prices of storage 35 years ago, it's a bargain.

Compared to the prices of storage today, when people are presumably buying the product, it's actually a bad price.

Sure, but this thread started with somebody pointing out that these computer prices are in line with Mac SE/30 prices. I'm just carrying that over to cover storage and it's even more amazing how much we get for so little money.

But software also fit in 640kB instead of 640GB.

It still can, but until very recently the motivation for keeping software small wasn't there. I'm still hoping the tide is coming in.