Good news and bad...
Bad (expected): Access is through the screen - much trickier than pulling the back off. It will require large suction cups, and there is a non-zero risk of breaking the screen every time you go in.
Good (expected): The RAM is indeed upgradeable The only difference from previous iMacs that have needed the display off is that you are pulling the display off a brand-new $5000+ machine, not a $1500 machine - previous iMacs that have involved taking the display off have tended to be low-end models).
Good (and at least somewhat unexpected): The SSDs are also socketed - this being Apple, they may be nonstandard, but every previous Apple nonstandard SSD has been solved (by OWC and others) pretty quickly. This should be upgradeable, although OWC didn't explicitly say so.
Good (and I hadn't seen this at all before, although it makes sense). The CPU is socketed, and it's Intel's standard socket. There shouldn't be any reason why upgrading between the Apple supported CPU options wouldn't work (buy an 8 core, later decide you want 18 cores - dropping a Xeon W-2195 in should work). It may (or may not) also be possible to drop in OTHER Xeon W CPUs.
Bad (although it makes sense): RAM is apparently quad-channel ONLY - the only possible upgrades are "pull out all 4 DIMMs and replace them". 256 GB may very well work - 64 GB modules of the right type are already available, although not as fast as the iMac Pro wants.
Bad (completely expected): The GPU (while socketed) is nonstandard. If the Vega 64 is available as a service part, it'll pop right in - but don't expect any other upgrades unless a new generation comes out with the same socket.
Overall, fairly darned upgradeable, although it's going to be a $100+ labor charge every time, because it's accessed through the screen. Buy what you need from the factory (not worth trying to escape an Apple Tax, except possibly for 128 GB of RAM), but it's easy enough to give it a midlife kick in a couple years.
Dan