Remove HDD and add SSD to Fusion Drive iMac

Hello,

I have inherited a 21.5” iMac, late 2015, model 16,1, 1920x1080, which came with the 1TB Fusion Drive option installed.

This machine is showing its age and I do not want to spend much money on it. It’s not being used for anything particularly advanced.

However, I’d like to remove the 1TB HDD and move over to (a similar size of) solid state storage.

A couple of requirements I have:

1). I need to be able to use Boot Camp.

2). I want everything contained within the iMac enclosure. i.e. no external drives.

I realise this means opening up the iMac etc., but I am prepared to do that in order to get a completely “internalised” solution.

I have been looking at some of the info and videos online and my understanding of the Fusion Drive implementation is that when I open the iMac it will contain both a conventional 1TB 2.5” HDD which is fairly easy to get to, and also a 24GB PCIe SSD which is a lot more difficult to access.

In an ideal world, I assume I would remove both these drives and put a new 1TB PCIe blade in.

However:

1). I have access to the tools required for 2.5” inch drive replacement and I am confident I can do it. Also, I can get hold of a suitable 2.5” SSD without breaking the bank (say 80 GBP).

2). The PCIe situation - particularly for this specific model - seems much more complicated. I don’t even know whether a suitable SSD is available, or affordable!

3). And in any case I am less confident about the trickier process of accessing the PCIe slot.

So, my inclination at the moment is to “break up” the Fusion Drive, open the iMac, replace just the HDD with an SSD, and thus end up with 2 SSDs on the iMac.

I assume the original 24GB blade will be too small to have as my primary OS drive…?

In any case, I am sharing the machine with a non-techie and I want them to be able to go on using 1TB of space without having to deal with extra drives etc.

So, I would then clean install MacOS on the 1TB SDD and just leave the 24GB SDD as a spare drive, albeit a tiny one.

I realise that the SATA SDD will be slower than PCIe, but bearing in mind all I have said above this seems to be the most cost effective solution, and will enable me to leave the PCIe slot well alone!

So, can anyone offer any advice as to a). whether this would work, or b). whether there are affordable, compatible, PCIe drives available?

Sorry to be so long winded but as I am asking for help I wanted to be as clear as possible!

All advice very very gratefully received…