“where are current compile instructions?” — it’s in the support document:
https://openthings.freshdesk.com/solution/folders/5000149652
The 2015 repository you referred to is an old branch that is not maintained anymore. The current OpenSprinkler firmware is a unified firmware and that’s in github.com/OpenSprinkler, not the rayshobby repository anymore.
Also, keep in mind that adding LCD and buttons and make the enclosure larger will inevitably drive up the cost of OSPi — I understand that many users choose OSPi because of its lower cost than OS 3.0.