iMac 27" (Late 2015) Fusion Drive to SSD

1. M2 drive you indicated is 2015 iMac - is NVME PCI-e SSD. The 1T Fusion drive has 24Gb PCI-e blade SSD; and 2T Fusion drive has 128Gb (?) PCI-e SSD. If you can get into logic board -- it would involve of taking apart every board/speakers etc. If you want to replace the PCI-e SSD with bigger drive, you would still want to consider SSD for the SATA drive (the HDD portion). HDD will fail (matter of when) and why not stick to SATA SSD (with less chance of failing, however, it might happen).
2. This option will be best for you bucks. Do not bother with rebuilding Fusion drive. Because smaller 24Gb, even 128Gb - it will have more wear and tear in fusion drive situation. Fusion drive is good on paper, but you have to go through some hoops when both drive decided to "spontaneously divorced" - the repair process is not worth the headache, to my taste. By getting a larger SATA SSD (OWC is quite reputable) to replace the HDD that the steps will be easier (still, not for casual DIY) and performance is significant. Sum it up -- use both SSDs separately will be my personal advice.

Then you should first check to see if there's any 3rd party software that might be impeding the performance of your iMac before you start replacing hardware.

Download and run Etrecheck. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Add Text button to include the report in your reply. How to use the Add Text Feature When Posting Large Amounts of Text, i.e. an Etrecheck Report

Before running Etrecheck assign Full Disk Access to Etrecheck so that it can get additional information from the Console and log files for the report:

Then we can examine the report and see if we can determine what's causing the problem..

1. There is NO internal optical drive in 2015 iMac. Actually since 2012, Apple had eliminate internal optical drive.
2. Make sure you have backups (even better if you have redundant multiple backups). Split it or not to split, will not matter - as soon as you put a SATA SSD, you have to use "disk utilities" to erase/format both drives (internal PCI-e SSD and SATA SSD). However, make a USB bootable drive of MacOS of your choice first, will save you a lot of time.
3. Since there is NO internal optical drive -- I would only offer that internal HDD is 3.5" that you have to use 3.5 -> 2.5 adapter for the SATA SSD.

One more, do you really want to open the iMac? The procedure of internal SSD upgrade, is not for casual DIY.. Some might suggest use external SSD..

Recommend to get and use Etrecheck per above instruction.

Run report, note details, and post the report into this discussion.

The OWC upgrade info shows parts and also instruction videos online, per links.

• iMac Upgrades per Model ID - verified upgrades OWC

https://eshop.macsales.com/upgrades/imac17-1

PS: Details on each product upgrade are covered per model; given knowledge of

how much the hardware can see in 'upgrade via replacement' parts, is published.

Another take on [ Hard Drive Upgrade Kit for 27-inch iMac 2012 - 2019 ]

Note they offer a Thermal Sensor for these models, so fans won't run fast.

OWC Complete Hard Drive Upgrade Kit for 27" iMac (Late 2012 - Current)

OWC SKU: OWCDIYIMACHDD12

OWC In-line Digital Thermal Sensor for 27" iMac (Late 2012 - Current)

OWC SKU: OWCDIDIMACHDD12

Actual page where this was found was sleuthed from a giant URL within their email.

More memory (RAM) is not a bad idea, that can help pre-load or otherwise move data

along toward the processors; in cue, to use more effectively. A small example, lesser:

my MacBook/Pro with tiny 240GB SSD which shipped with 8GB RAM, does better with

16GB RAM, and was easier upgrade. Even though bottom of MB/Pro needed removal.

Their 'kit' offers several extra tools that may be harder to find, at fair cost, for these projects.

FWIW, I have a Late 2015 5K iMac with a 2TB Fusion drive. I also have an iMac Pro with an SSD. Personally, there's not enough of a speed difference I would bother replacing the Fusion drive until it breaks.