I've never tried it on large biomes, but the seed 'wakeup' is pretty good for mountains near plains. I'll just say this much, and let you see for yourself what the seed has to offer:
When you spawn, there's a large mountain in front of you, and behind you lies a vast plain. Further north there's a larger mountain range with spruce trees.

I love this seed, it's so much fun to play on

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I found it using AMIDST.

I have a PC and I doubt it will work at all.

I have a seed for you to try... Its not exactly what you are asking for, but it does have some nice large hills and mountains and cool overhangs. I am using it on xbox so idk if it will be the same on pc if thats what you are using. The seed is -543689473

I don't want the sahvanna, I want it to be an extreme hills biome

The plains and mountains are connected. There is a 4-biome intersection. Swamp, desert, plains and savanna plateau (the mountains).

Same seed on large biomes, you spawn on the edge of an huge plains biome. Don't know if it s connected to mountains, possibly is because the plains are huge.

Okay, but I meant the mountains and plains biome to be together. Plus the seed is outdated

Got a perfect seed for you. A seed that I have been using since Minecraft beta 1.8. It has gone trough some major and minor changes, but it is my favourite seed. Not a large biomes seed though.

Seed: 108181935

Even has everything you wanted:

Spawn at plains (wool = spawn):


Next to this (continues further with more awesome stuff):


Map from around the spawn (explore the rest for yourself):

I try but they will most likely have outdated seeds.

You could google it? I don't remember what it's called, but there's this website with a ton of seeds with really cool things!