One more from my private seed collection; finally took the time to actually play it. Tested on 1.11.2.
Seed: Insel der Pilze
Yes, you didn't misread that, it's literally "Island of mushrooms" in German
Spawn is on a little island with not much on it, but in the west you can see an adjacent tiny island with four trees. I would suggest harvesting at least one tree and crafting a boat and bowls. If you plan starting a tree farm on the mushroom island, also collect any saplings and half a stack or so of dirt. Then man (or woman) the boat and head east-northeast to northeast. It's quite a distance, around 600 blocks.
The mushroom island is connected to a large desert island by a narrow isthmus. Underneath is a mineshaft that connects to an underground ravine, and also has a connection to the one deep-going cave system on the mushroom island. However, that connection is tiny and possibly easy to miss even by the seasoned spelunker. Here's a location tip:
The tunnel connecting to the ravine and the mineshaft is in a diorite cluster under the ceiling, in a bend before a circular room with water on the floor and a wall in the middle.
You will hardly encounter hostile mobs underground due to being in a mushroom biome nearly all the time, but the cave spider spawner WILL be functional Should you find yourself bewildered by the sounds of guardians deeply under the earth, it's because there is an ocean monument right off the island's north shore and that cave system is quite huge