This is a great Mushroom Biome seed its name is Greta2006 and should be played on Large Biomes. (otherwise you start in the middle of the ocean and have to explore to find a much smaller mushroom island.
Underneath the mushroom biome is a massive abandoned mineshaft cave network. I didn’t finish exploring it, but it is really large. You can find it digging downwards pretty much anywhere but if you want specific coordinates dig at X: -989 Y: 65 Z: -412
A couple of surface diamonds in the mineshaft system at X: -1044 Y: 11 Z: -408
There is a stronghold below the mushrooms at X: -1105 Y: 33 Z: -289 which will take you to the end. The stronghold library is at X: -1075 Y: 30 Z: -285