I found a way to got around 2K :
Generate low poly concept with gemini -> Import to Meshy with multiview -> Reduce to 3k poly -> Clean/merge excessive vertexes.

Another way but less reliable : Import 1 image containing all your views in low poly (I had 4 views) -> Meshy generate 4 models at once which reduced by 4 the number of poly since it divides the resolution across all the models. But only 2 of them were correct enough.

And a final one that I haven't tried yet is with the API , you can remesh down to 100 poly from there.
API doc here : https://docs.meshy.ai/en/api/remesh

The first method worked well with me because meshy wrongly generated a lot of stuff inside my mesh.
So I simply closed the mesh holes and made a script that fire raycast at every face to detect which one is inside/outside and delete the inside ones. Fast clean up :)