at least 12GB of RAM,

Doesn't exist and frankly not necessary. At present nothing needs this much RAM and I say this as a power user who runs a local media server and is familiar with concepts such as running Plex with remote transcoding on devices such as the Shield. The only things that come close to this much RAM might be recent generation gaming consoles like Xbox or Playstation but they don't run Android and aren't built for streaming, remote controls are third party accessories.

Other problems: I'm unaware of any devices even without the high RAM request that exist that fulfill all of those asks. Particularly 4k 120hz seems to be something no one has any interest in putting in a streaming box despite plenty of them having HDMI 2.1. I checked and even Apple TV 4K's do not support 120hz despite having the right hardware for it.

Fact is commercial streaming services which these devices exist to cater to have no interest in 120fps/hz content, even 60fps with dolby vision is kind of more bandwidth than they like using and they upcharge premiums for it.

Basically your choices are (all these assume giving up the RAM ask):
* Run two devices, one for local streaming of things like games and 120fps content, one for watching streaming services that's Android TV certified and will get you 4k content from them (at only 60fps). Cost is probably going to be north of $300. You can with this option get a mini-PC and kind of hack together a game streaming solution and if you want you can put 12GB of RAM in it but you won't need them.
* Buy something that ticks most of the boxes but either give up the ability to stream 4K content from commercial services (because no Android TV certification) OR give up the 120fps demand. If you're willing to give up the 120hz you might look into the Dune-HD premier 4k pro which has full Android TV support and certification for 4K streaming from all the major services, does 4k 60fps, has very impressive specs for a streaming device (top of the line) and is $200 and you can add an SSD to increase the storage from acceptable 32GB to well over a TB if desired. Otherwise if you're willing to give up 4K from commercial streaming services and get only 1080p from them they have a few 8K devices that do have support for 120fps but the price doubles and if you want 4k on the major streaming services you'll need another device.

Other options if you're willing to give up the 120hz demand would be the Nvidia Shield Pro. As to a remote that controls everything, I believe the Dune-HD remotes do have that capability but honestly your wants are really specific and high end. Most people would use two boxes if they HAD to have all you've asked for.

You might also consider waiting to see if Valve does release a streaming device like they've teased. If Valve does something like that it might finally be the golden one that has 120hz support though you're still not going to get 12GB of RAM. But you're looking at waiting a year at least I think even if that does materialize and it may not.