[2020] Is there a way to disable all mods but still keep Loading Screen Mod active?

You could copy the Loading Screen Mod to the local mod folder and just use the -noWorkshop launch option. If it's the only local mod you have installed, you won't need the -disableMods launch option. Just keep in mind you'd have to either remove the local mod or unsubscribe the workshop version when you take -noWorkshop off again. And if you stick with having it locally installed, you'd need to manually update it.

Local mods go here: C:\Users\"YOURUSER"\AppData\Local\Colossal Order\Cities_Skylines\Addons\Mods