Does the 2019 iMac use proprietary SSD controllers or connectors? My old workhorse 17" MacBook Pro (circa 2011) finally gave up the ghost after 8 years of reliable service (may it rest in peace), and I'm looking into a 27" iMac to replace it.

I want to buy the high-end model iMac with the fusion drive and make the following upgrades:


Replace the 128GB SSD with a 1 or 2TB SSD.
Replace the 3TB spinning HDD with the largest SATA drive that will physically fit inside the case (hopefully a 12 or 16TB drive - I need lots of storage).
Ditch the Fusion Drive setup in favor of two independently formatted internal drives.


I understand that this is complicated surgery for this model and I have the skills necessary to perform the work.  But since my previous machine is so old, I am a bit out of the loop on newer Apple hardware.  I know that Apple was using proprietary connectors for their SSDs a few years ago (they were first to market with PCIe-based SSDs before the NVMe spec was finalized in the PC world).

So my question is, does the onboard PCIe SSD connector on the 2019 27" model conform to one of the industry standard M.2 form factors, or does Apple still use their own proprietary blade-style SSD connectors?

Also as a bonus, what is the maximum height clearance for the 3.5" SATA hard drive bay?
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.