I’ve done exactly this task myself at my day job. I really wanted to get around to writing up a long blog post about it all, but never had time.
I wrote a small Python server that took standard TMS format queries (something like /z/x/y.tile), read the corresponding tile out of an MBTiles container database file, and returned it. I also did some work to handle multiple MBTiles files at different zoom levels around the world (such as a worldwide tileset up to zoom 8, and specific areas at zooms 9-13).
For terrain generation, I used the linked cesium-terrain-builder utility, and actually filed a PR to enabled outputting the contents to MBTiles format:
For our own use, I generated terrain data using the free ALOS30 worldwide terrain dataset:
https://www.eorc.jaxa.jp/ALOS/en/aw3d30/index.htm
Had to do some preprocessing to get it ready for use by CTB, but ultimately got some very usable data.
I sadly don’t have time to put that blog post together, but if you’ve got questions, please ask and I’ll be happy to try to provide more details.