iMac 4K, 2019, 21.5-inch SSD Upgrade possible?

You might be better served, budget wise, by getting an external SSD, cloning your current drive to it, booting and running from the external SSD. I recommend drives from OWC (MacSales.com) as they have proven reliability, excellent customer support and warranties.  Consider one of these depending on your budget and needs: 

OWC Envoy Pro FX Thunderbolt 3 + USB-C Portable NVMe SSD

OWC Envoy Pro SX Rugged Portable NVMe SSD with Thunderbolt/USB4

OWC Envoy Pro Elektron USB-C portable NVMe SSD

Envoy Pro EX with USB-C or USB 3

OWC Envoy Pro SX - up to 2847 MB/s 

However, for your model I recommending OWC's customer support and get their recommendation for the SSD model that would best suit your iMac model, work requirements and budget.  They may have additional models other than those I've shown above.

I don't work for them but do have 7 external SSD from OWC and amy very, very satisfied with them.

Yes, but it requires complete disassembly of the iMac.

Apple Stores won't do this for you. Apple Authorized Service providers may do it for a fee.