Use this tool to download mods.

https://github.com/VovoloGames/WorkshopDL


For now it works. As usual you should always back up mods you like in case the tool stops working in the future,

Never heard of this, does it work for many GOG-games?

Does it give you offline-installers if you want, to manually extract into the correct folders, or is everything online/automatic?

I would really like the mini-map to be bigger, or get some large red warnings for hairpins (like on traditional rally games) - might there be any such mod for Wreckfest?

It doesnt work for every game but it works for wreckfest. There is a lists of games that the program supports once you launch it but there are games that work and are not in the lists so it's better to test each game. There are 4 different download providers so if one is not working try the other one.
It doesnt give you an installer but it gives you the files in a .zip format which you have to extract and then transfer them usually in the installation folder of the game.
About your last question can't really help you. You have to go to the ste@m workshop and look for it yourself if it exists.

If you actually download any mods I would advise you and anyone else to back them up because you never know when this tool might stop working.

Thanks for this. Useful information!