Well, yeah... I'm a little bit, deflated?, TBH.
It would seem that the culprit was, as discovered by zybex, this thing called "Smart App Control"
https://www.howtogeek.com/smart-app-control-windows-11-explained/Smart App Control is a feature within Windows Security that provides protection against untrustworthy apps. When you try to run an app, SAC analyzes its credibility using Microsofts Intelligent Security Graph, a cloud-based security service. If the service is unable to make a confident prediction, it then checks the app for a valid signature (which identifies where the file originates from and if it's a trusted source).
Apps from known publishers or with valid signatures will run as expected. If not, the program is blocked. Theres no manual override option, nor can you add an app to an exemption list.
Since turning this thing off, installers are insanely fast. MC's goes through its thing so quickly, I don't have time to read what it's doing!
This is the WOW! factor I was expecting, you know, when you press the 'Power On' for that first time, and it never came.
Getting all that software I use daily back in place, every, feckin', installer, slower than a slow thing... Not what you expect from your new toys, right?
Smart App Control checking everything online? FFS

I didn't even know this was a thing. The very least Windows could have done was let me know this active and give me the option to turn it off.
In amongst all of this, I learned that my ISP's router was having "provisioning" issues, whatever that means, so had to endure the phone call to India to get that sorted. To be fair, it seems to be sorted, I just get disheartened after waiting on hold for some time, to eventually be connected to some foreign land, talking to someone called Albert?? whose English is so accented it's hard to understand, plus, you can hear another countless calls going on around him, almost like you have a crossed line (who remembers those?) and my eyes roll to the ceiling, do some quick breathing exercises, thinking, the quicker we get through his script and I get passed to level 2 support the better...
But, credit where it's due, "Albert" could see there was a problem, and seems to have been able to fix it remotely, so no need to book an engineer visit. Way to go, Albert, two thumbs up

There now... Having just unloaded to all in the PC's and Other Hardware forum, and doffed my hat to zybex, I feel waay better now. Like a child, I've just run the MC installer many times in a row just to marvel at the speed it does its thing here. I can just about make out the installer stages... Lovely.
Made me wonder though, that Mac Mini of ET's..., benching in MC another 4 to 5k above this? That must be soooo fast? Do you even see the installer on screen for an auto update of MC?