I've finally added as much plaster cloth as I expect I'll need, and have moved on to priming the base before painting it. Priming made it look unfamiliar, but also like I've actually *done* something. No more odd mix of pink foam, black artboard and white plaster cloth.
The painter tape on the track serves as motivation to keep moving on this layout. I don't want to lay down and take up the tape again and again, so I want to do as much work requiring the tape as I can (plaster cloth, priming, painting, adding turf, etc. It's made me move much faster, so I can get back to playing with trains.