Linux Migration - Current Queries Edition (March #1)

This is the latest in my ongoing series on investigating my setup for migrating to Linux. I've got 2 other posts that were supposed to go up at the start of February, but I've been procrastinating on polishing them up for posting...

In any case, today's update covers where things are at in general, and the outstanding barriers to resolve before pulling the trigger.

Current Status

If push came to shove, I could drop each of my machines off the internet, do a final backup of critical files, then do a migration tomorrow. 

BUT, right now I do have machines that are largely functional + by and large already well set up for how I'm using them. So, I'm really in no rush... yet...

Outstanding Queries:

Hence why I can afford to spend time looking into the "right" way to solve the last few pending unknowns (like the following):

1) What's the best way of making a full copy of the relevant drives on each machine, such that in future, I can just plug those drives in, and get back my old Windows environments, just as I left them

i.e. I must be able to do plug-and-play booting of WIndows from what will likely be External HDD's at this point (unless using USB really turns out to be too slow, and I migrate again to use an internal HDD instead...  (In any case, relying on SSD's for this task is out of the question, due to the data-corruption-at-rest-when-unplugged issue)

That's the general form, that applies my two desktops.

2) For my laptop, I have a slightly different variant:

How can I make one of those disks, where it includes both the SSD / NVME "500GB Windows C:\", AND the HDD ("1TB Data D:\") drives and hosts them all on a "2TB HDD"?

3) I was originally wanting to be able to this without having to burn a separate / dedicated ISO to do this task (i.e. "clonezilla ISO" or "rescue<whatever>") 

That had been motivated by not wanting to lose the LiveUSB setup I had just spent time customising to get it to the point where I could do basic testing with it... Then again, now I've verified this works, so this last part is less important...

4) How to make the bunch of small visual tweaks to the combination of KDE + GTK themes that I want to make. In particular:

* Give Dolphin chunkier + colourful icons for its toolbar

* Custom window decoration theme to incorporate all the visual styling I want

* How to force the GTK toolbars to be thicker + stay always visible

5) Failing to get the booting working above:  Is there any harm to installing Linux to a portable hard drive, plugged in via USB 3.0, and just running my machines that way until I'm satisfied I have the Windows backups sorted. 

If not, that'll be the way I go - except for the laptop, where it's now likely that I just switch it fully once I do a backup of the data only (with no way back).

About that Laptop...

Honestly, it's a lower priority machine for me these days, but has been running bad under Windows these last few months (I.e. now has constant loud fan noise + choppy mouse input), AND as the second February post would say, turned out to be much better than expected under Linux  (well... OK, I haven't checked the battery life!)

Heck, things are now so much nicer there (sitting on the LiveUSB, sleeping for the last few weeks with occasional logins from time to time for light browsing + tinkering), that it has now turned out to be my likely first candidate for migration instead, whereas it was originally going to be the last (so I had a "rescue box" I could always rely on when the other 2 got a NTFS drive into a troubled state...  Also, probably as my "Disney+" jump box should it allegedly refuse to run under Linux as I've been reading...)

(Yes, I'm generally enjoying having it like this too much now)