after spending about a week looking for a good mushroom biome i found a seed that is pretty good
the seed is 7319697500799939733
just look behind you(you spawn on a island and the biome is right behind you)
(lol good luck finding trees though)
(spawn point view of island)
I could use this for my new minecraft series maybe..... I would have to cheat maybe.....
lol just swim backwards from the spawn point for a minute or two eventually you'll reach a snow biome where you could get a few saplings and some wood (lol even though i tend to use tmi ) the snow biomes i think 300 blocks/meters away from the spawn opposite from the main shroom land
I made this account to simply tell the OP to dig straight down from your spawn spot! You have no idea what a jackpot this is.
Digging down from spawn spot you will hit a cave (I did for 6/10 times I loaded this world, sometimes you wil just hit the stronghold), keep digging down don't explore the cave and you will hit one of the largest strongholds I have ever seen. The part you dig into is a bit broken but if you follow enough stairwells down you end up in exploring many many chambers, fountain rooms, libraries, portal etc Also a few dungeons off cave systems and there is a cool ravine that cuts some of it too.... been a few days and still exploring.
OMG I've never had a huge mushroom island and a huge stronghold so close before
(I'm too lazy but someone with a good xray mod should take screenies and upload it so ppl can see the size.)