Ending Fusion-Drive with SSD upgrade, while keeping existing macOS?

Hi!

My iMac 19,1 is painfully-slow, due to it's Fusion-Drive. I have read a lot on these and other forums about disconnecting Fusion-drives, to replace the PCIe/NVMe and/or HDD with new SSDs (PCIe/NVMe + SATA).

I've read that it's possible to disconnect the Fusion Drive either by booting from a USB media, or Time-Machine backup, or by being fast at POST and, invoking the Recovery Console, finding the exact designation of both disks, and then using this and following the command-line syntax to 'break' the Fusion apart.

Then the iMac can be opened up to replace the HDD with an SSD most commonly, sometimes the PCIe/NVMe. I have read that is you are only replacing one drive (or want to dismantle the Fusion drive but keep the same hardware), then there can be issues with the Mac trying to 're-fuse' (boom boom), these peices of hardware - so the manual decoupling of the Fusion drive is important. Whereas, if swapping out both drives, then clearly there is 'nothing left' to work with, that the Mac would now work with.

A curved-ball suggestion seems to be running a new PCIe/NVMe drive in an external enclosure over Thunderbolt 3 as one's bootable macOS drive, potentially leaving the existing SSD & HDD where they are (but decoupled as a Fusion Drive), to just become 'some storage'. I think this setup would be fast than anything bootable that incorporated a replacement SATA SSD for the SATA HDD, as the limitation is SATA.

I think potentially, the 'external enclosure with the PCIe/NVMe drive Thunderbolt-3, may even be comparable to a motherboard-seated NVMe? As, what I understand overall is that the read-writes on 'whatever SSD, on any interface', maybe the more limiting factor, than any specific interface. So 'ultimate speed'sounds like it therefore comes from 'any' RAID-array of NVMe drives, in an external enclosure over Thunderbolt 3. Some has even created a RAID between a motherboard-mounted NVMe and one in an external enclosure! - that blows my mind, and I have no idea 'how'/what would be efficiently controlling the overall data read-writes, so the whole thing is balanced for speed/volume/interface!!!

So yeah 'All That'. BUT ...what scares me though, is that EVERYONE (bar one, @SirLuc ) casually refers to '...and of course you do a reinstall of a fresh macOS partition.

There is no 'Clone the bootable macOS partition spread across the Fusion drives, swap whichever drives one fancies, and then 'import' that Clone to (by default) the new, motherboard-mounted MVNe.

Why? As, surely the the purpose of Cloning is to potentially do EXACTLY this, with dissimilar hardware. I cannot beleive it is always 'easier' for every single person doing an upgrade, to start-over from scratch.

Yes, I know in principle 'it's better' to start afresh - things may run faster, for a brand new OS, more stable, for 'having less going on', and for all I know the overall it may be 'faster to do', even having to download the whole Adobe master suite, re-install 1password to get all my passwords back, etc etc.

But what if I just like 'how it is', and want to 'keep it that way'? So for all those who are like-minded, but as far as I can tell have 'never asked' (or at least 'here', and been answered'!), what I want to contemplate is:

• Cloning the bootable macOS partition to a new, large, fast NVMe drive in an external enclosure, via Thunderbolt.
• ‘Hope’ that this drastic change in underlying storage hardware/array from PCIe/NVMe on the motherboard + SATA SSD. Vs PCIe/NVMe via Thunderbolt NVMe does not crap-out the macOS logical partition.
• i.e. I could boot the Mac from this Thunderbolt-connected SSD and it would retain all my functional software, subscriptions etc. And I would see a clear speed-bump. Can I ‘test this’, while the pre-existing Fusion-Drive remains intact?
• Ideally, to then be able to dis-associated the internal Fusion Drive into the NVMe SSD and the HDD - but, what it ‘left’/what does one ‘get’, for doing this? Is the motherboard-mounted NVMe still bootable, and the HDD become visible as a ‘data-drive’, or is everything ‘lost’? I believe it cannot be re-Fused, so it would be nice to know what the ‘result’ of disassociating the Fusion drive ‘looks like’?
• Ideally, then to disassemble the iMac, remove the existing small NVMe SSD and replace with with the not from the Thunderbolt-connected External enclosure, onto the motherboard. Would it ‘plug-and-play’? Is a cloned, bootable macOS partition ‘hardware’ agnostic enough not to care either about the change in storage hardware, and interface?
• Ideally, to replace the HDD with an SSD. Either, as a separate data-drive, or possibly a bootable Windows drive.
• Can a Parallels VM be spun-off like this? Or, can a physically-separate Windows SSD be set up as ‘Bootcamp’, so bootable separately, AND be accessible from within macOS via Parallels?

However - I am not clear that Time Machine, Carbon Copy Cloner or GetBackUp can 'confidently' achieve any, some or all - of the above.

It may be possible though, as I think @SirLuc 's post in reply to one of the 'Fusion-drive replacment' type articles, says so. He used a bit of software I've never heard of - Superduper - to 'export, swap-out hardware, import back', and it seemed to work He/She.

I think this was 'only' Cloning the HDD part of the Fusion-Drive and 'making that work' however - with a side-helping of succeeding in porting a Windows partition care of WinClone too.

So that's my challenge - how to do this'yeah, be gone Fusion-Drive' hardware changing out stuff, without a fresh install of macOS.

OR if there is a 'perfectly merge everything' option that builds on a fresh install and 'grabs' from a 'backup' or a 'clone' then fine - but I cannot confidently 'find it', this far down the rabbit-hole.

Thank you!

Damian