Does the 2019 iMac use proprietary SSD controllers or connectors?
These models support a 7 mm tall SATA III (6 Gb/s) hard drive for the primary storage (using the 3.5" size as opposed to the 2.5"), and getting the Fusion drive means you'll also get what's called a small "blade" SSD via a PCIe connector. Without this you can't attempt what you're wanting to do.

So, in summary:


Primary storage uses a 3.5" 6 Gb/s SATA 3.0 connection
Secondary storage uses a NVMe PCIe "Blade" connection


However, getting a compatible "Blade" SSD is not as easy as walking into any computer store and buying one. You will need to purchase one from a vendor that specialises in Mac hardware to ensure you get what you need.

I haven't actually seen inside a 2019 model 27" iMac yet, so can't give you a definitive answer in terms of height clearance. 

Recommendation

Before biting the bullet and ordering one of these machines, I would take a look at the following to ensure you can get what you need and also do what you want. 


How to Upgrade or Install a SATA Drive (SSD or HDD) in the Main Drive Bay of a 27-inch Apple iMac (2012 - Current)
How to Upgrade or Install a PCIe SSD in a 27-inch Apple iMac with Retina 5K Display (2019)
OWC Solid State Drives For iMac 27-Inch (Late 2013 - Current)
OWC Complete Hard Drive Upgrade Kit Including Tools for all 27" iMacs 2012 &amp; Later


Since you have the skills necessary to perform this upgrade, I would focus on ensuring you can actually get a compatible SSD with the storage you need.

NOTES: 


For something like this I also recommend contacting OWC to discuss your requirements.
There would be no need for you to get the 3TB capacity Fusion drive in order to be able to do this, unless you want that capacity for whatever you intend to do with it later.


Disclaimer: I have no affiliation whatsoever with OWC.