I'm trying to figure out what the current workflow is for getting custom meshes and textures into TM2020. I have some ideas for tracks I want to make, and I have professional modeling and texturing tools, but I can't for the life of me figure out how to get that stuff into TM2020?

Do you still have to import everything into TM2 first, then export from there and import to TM2020? Has someone come up with a better workflow yet or a better tool? Additionally, if I get custom models in, how do I get custom textures to embed in my map for custom objects/items?

blame nadeo man

You can use the NadeoImporter and ConvertAssistant to get make an item for TM2 using some external tools.

But instead of going into TM2, use the Mesh Modeler inside TM2020 to import the .Mesh.Gbx into a new item. The UVs will be kept, but you'll need to select the relevant faces and reapply new materials.