Upgrade the NVMe on 2019 27" iMac

If you are going to open up the iMac, then why not just get rid of the hard drive and go with a single large NVMe SSD from OWC? It will simplify the process and gives you a faster SSD since a SATA SSD is limited to 500MB/s. The OWC Aura SSDs are at least 3x+ faster. You do not want to use a Fusion Drive with two SSDs as you will end up wearing them both out since SSDs have a limited number of writes available to them.

You need to be very careful when opening the iMac since the power supply has exposed circuitry that can give you a deadly shock. The power supply can retain power for many hours after power has been removed from it. The iMac LCD Panel is very delicate especially on a 27" model plus there are several very delicate cables & connectors you must deal with that can be extremely hard to see and reach. Make sure to watch the OWC upgrade videos to get an idea of what is involved. Keep in mind it always looks easier in the videos than it is when you are actually doing the work yourself.

No mater what make sure to have frequent & regular backups since SSDs can fail at any time without any warning signs.

None. You have a Fusion drive, replace the HD with a SSD. Even better, just buy on the these:

https://eshop.macsales.com/shop/owc-envoy-pro-ex-thunderbolt-3 and then clone the internal drive to the SSD and you are in business. No reason to open and possibly destroy your iMac.

Just in case you have Applecare, you best NOT open the Mac yourself or your warranty will be voided. Only Apple or authorized service can do that without voiding the warranty.