Hey everyone, after a while of trial and error I finally found a reliable way to upload Spintires maps onto the Steam Workshop !
That's the great news. The bad news, however, is that the process is unfortunately not fully automated and requires some user intervention for each new map you download. So let's get into it.

The first thing you need to do is to head to your Steam Workshop folder. The path should be something like : C://Programmes/Steam/steamapps/common/workshop/content/263280
It's entirely dependant on where you have installed Steam on your computer, and this is where Steam downloads the Workshop items you're subscribed to, 263280 being Spntires game ID on Steam.
Inside that 263280 folder, you will find a lot of sub-folder all composed of numbers as well. Those are the individual Workshop items.
You need to note down the precise path to that locasion as well as the number of all the sub-folders that are map mods (you may want to sort the folders by modification time if you have a lot). Open them to see what's inside, map mods should have folders called "level" with a lot of .dds texture files.
Note the name of such folders and keep it somewhere close.
Edit : You can also find the mod ID on Steam, either by showing the URL bar (in the settings) or by doing right click on the workshop page and doing "copy page URL", the final part of the URL being the ID you're looking for.

The second step is to go to your Spintires installation folder, which should be in .../Steam/steamapps/common/Spintires.
Then enter the User folder and create a new file called UserMediaPaths.xml (you can do so using your basic Notepad editor).
Inside that folder, create two XML beacons like so :

<DataSources>

</DataSources>

Copy the following line between these two beacons, as many times as you have maps, and complete the path with the ID number you noted down earlier :

<MediaPath IsOptional="true" Path="<your path here>/Steam/steamapps/workshop/content/263280/Mod ID here"/>

A relative path also works, as such (credit to g13ba), assuming that you have a default Steam installation folder structure :

<MediaPath IsOptional="true" Path="..\..\workshop\content\263280\Mod ID here" />

Additionally you can also add a comment line above with the name of the map and the mod author (or anything else of importance), to help you keep the file sorted, like so :

<!-- This is a comment. Map name : ####### Map author : ####### -->

Note that with the IsOptional tag, the game will not crash if you unsuscribe from a map but forget to remove the tag from inside this file.
You are still, however, forced to add a new line for each map you download, so it can appear in-game.

And with that, you can now properly link your Workshop maps to your game !
Unfortunately that is the most straightforward solution I have found, and the one requiring the less user action, though far from perfect.

Check my other guide on how to upload the map you have created onto the Workshop :
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2954541065