[2020] Assets gone

Do you see them in the Steam Workshop category in the Content Manager? Or is that empty too? And do the default mods like Unlimted Money and Unlock All show up properly?

Check to make sure you don't have the -noWorkshop launch option running. I've left it in more than one time and forgotten about it. Go to your games Library in Steam, right click on Cities: Skylines and select Properties. Then click the "Set launch options..." and make sure it doesn't say "-noWorkshop" in there.

For the Content Manager I'm specifically thinking of the category highlighted here:
https://i.imgur.com/VrnmAP5.jpg

Sometimes things show up there while not being in the other categories. When they show up there, it's usually a Steam connection issue, but you've already tried the things that usually fix it. If they show there, we might need to do a clean install of Cities to get them to show up correctly.