Have you gotten your iMac 27" yet? I'm still waiting for mine after ordering from Machines, same specs just that I opt'ed for the 1TB rather than the 2TB (maybe I should've).

I've been watching closely on many youtube reviews, and other than OWC, there ain't another other brands that you can max out 256GB.
Corsairs like these "https://shopee.com.my/CORSAIR-Vengeance-SODIMM-DDR4-2400Mhz-2666Mhz-Notebook-Laptop-RAM-(4GB-8GB-16GB-32GB)-i.145078604.2585110051", should be compatible as watching LinusTechTips on his upgrading of their iMac27' review.

I'm planning on getting probably 32*2 plus the existing 4*2gb ram on my mac. and thinking of getting the 5k Ultrafine display(Not a prority, but thinking), and also potentially getting an external NVMEm2 enclosure as my scratch drive to edit videos.

Hahaha, apple's official site says that 27" iMac supports up to 64GB lol.
But yeah, that's what I meant, there's no other vendors making 64GB Sticks (yet). Though it is tempting to think whether can this iMac support up to 256GB lol.

I haven't pulled the trigger on getting 64/128GB Corsair rams yet, HECK i haven't even unboxed my iMAC yet