Alright y’all, here’s how to stream/load ymaps, and convert some map XML outputs to XML for ymaps, for RedM 
Others have already been doing this, but I’m posting a general guide so that anyone can start messing with it, as I had read Disquse’s brief example and post from a while ago (here), but could never figure out how to do base ymaps and I didn’t dig into it for long because of that. This is relatively simple, so go for it if you’re curious!
First, create an fxmanifest.lua that only contains the following:
fx_version "adamant"
game "rdr3"
this_is_a_map "yes"
rdr3_warning "I acknowledge that this is a prerelease build of RedM, and I am aware my resources *will* become incompatible once RedM ships."
Then create a folder called “stream” inside that resource folder. Inside that folder, add any ymaps that are in the modern xml format (compatible with both FiveM and RedM). Just make sure they’re plaintext XML in that format and are named .ymap
Start the resource and you should now have those ymaps loaded at the coordinates specified in the files.
Creating ymaps can be done using a handful of methods, such as single player mod map editors, but RedM compatible scripts like spooner from kibook also output ymaps in XML:
However, if you get maps in the older XML format, like from single player map editors, you will need to convert them using something like the following tools:
https://redm.khzae.net/lambda2ymap/
You can also use other third party map sources, primarily for testing (subject to any licensing of course. I and CFX do not condone any pirating or use of resources against their licensing or the TOS), and convert them if needed.
Props to Disquse for their original announcement, kibook for spooner and lambda2ymap, and Lambdarevolution for the plethora of maps made from their map editor by the community that I’ve used for isolated testing.
I’ll be continuing to dive into streaming now that I’ve had some success finally. I’ll share further progress on other types of content streaming later have fun y’all!