Upgrading IMac 2019 27” Retina to M2 PCIe instead Fusion Drive, as a the only disk

Plug this OWC Express 1M2 external SSD into your Thunderbolt port (via USB-C), install Sonoma on it, and then use Migration Assistant to transfer content from your last Time Machine backup to it. Then change your System settings : General : Startup Disk and you are good to go.

The caveat of using a non-Apple drive is any future operating system update/upgrade may want to write new drive firmware to an Apple branded drive, and the installation will fail using third-party storage solutions.

The SSD connector on iMacs, whether only SSD, or Fusion drives is beneath the motherboard and it requires completely disassembling your iMac with substantial risk of collateral damage. A fast, external SSD simply avoids this grief.

Other World Computing sells internal SSDs that are compatible with the 27" 2019 iMac.

OWC Solid State Drives For iMac 27-Inch (Late 2013 - 2019)

Installing one might be a lot of work. You'd need to

• Make external, bootable backups of the system on your Fusion Drive. (Preferably on two or three drives.)
• Remove the existing internal HDD and SSD and replace them with the OWC one. (Not a job for the faint of heart; might be better left to a technician.)
• Boot from one of your external backups, then initialize the new internal SSD and clone the backup onto it.

Since your Mac has USB-C / Thunderbolt 3 ports and USB-A (USB 3.0) ports, it might be easier to get an external SSD. You can get Thunderbolt 3 / NVMe external SSDs that run nearly as fast as internal ones. There are also USB-C (USB 3.1 Gen 2) / NVMe SSDs that are a bit cheaper than the Thunderbolt 3 ones, and faster than USB 3 / SATA ones.

Then the procedure would be

• Attach the external SSD
• Erase the external SSD (using Disk Utility) and clone your current system onto it (using Carbon Copy Cloner or SuperDuper!)
• Make the external SSD your startup disk
• After restarting, eject (or ignore) the Fusion Drive, if you don't want to use it

That is a major undertaking with extreme risk since just about every internal item must be removed since the NVMe SSD slot is located on the back of the Logic Board. In addition to the tricky maneuver to remove & reinsert the Logic Board (it is awkward & tricky), there are lots of very fragile & easily damaged cables & connectors. Plus the Display Assembly itself is easy to damage as well.....the LCD Panel can break under its own weight if it is not kept completely vertical. @Servant of Cats suggestion for OWC is a good one if you insist on pursuing the internal drive upgrade since OWC also has installation videos for the process, but keep in mind those videos make things look much easier than it is, but it can be done.

Most likely the internal hard drive is wearing out or starting to fail...assuming the slowness is not caused by a software issue.

I would recommend using the external SSD as described here. Fusion drives can sometimes "split" then the repair process is extensive and laborious. Using the external drive will avoid ever having to deal with that problem.