What alternatives did Einstein ponder to supersede quantum mechanics? It is well known that Einstein was unhappy with quantum mechanics and it is also well known that he worked on new ideas.

Is there surveys or books or research notes or lecture notes that talk about ideas Einstein considered to supersede quantum mechanics?

 A: In the early '50s David Bohm devised a hidden variable theory which exactly duplicated the results of QM. Bohm's theory had a lot of support from other prominent physicists, so Einstein must have been aware of it. John Bell was very impressed by it, and believed it was worthy of a place in standard text books. What Einstein thought of it isn't recorded, so far as I know, but he must have been impressed as well. After Einstein's death, other physicists, notably Tim Bowyer at City College, New York, have developed ideas which replicate QM using classical theory. Nobel Prize winner Gerard t Hooft believed a deterministic cause and effect mechanism underlied Quantum Mechanics.
A: The article on classical unified field theories on the wiki is a pretty good overview.
Tl;DR: basically GR says that gravity is caused by the geometry of spacetime. We do not directly perceive this geometry, and the difference in what "we see" and the actual geometry gives rise to the various effects we call gravity and time.
You can extend this basic concept with additional dimensions to create additional actions. This is what Kaluza did by adding a 5th dimension, which provided EM-like interactions. This was then extended to become Kaluza–Klein theory.
Einstein was basically working along these lines for the rest of his life - as were many others including Eddington and Schrödinger. However, they were being outpaced by QM, which was adding new phenomenon like isospin and such, complicating the theory faster than it could be solved. When QFT/QED came along, the quantity of work on classical theories basically died out.
But, everything that is old is new again. String/m-/brane/whatever is a modern retelling of these basic concepts.
