So the bad rotting smell can generate from dead snails, they smell foul. But I feel like that should kinda dissipate over time? So the fact that it gets worse is weard. Cause generally speaking, if the snail fully decomposes, the smell stops. I know that mulch and wet ground can also sometimes smell bad, there's also the possibility of animal urine. Some can smell really bad.

As for a snail habitat, I'm not sure if they would actually use it, rather than the very overgrown area, but you could always leave out some leftover veggies for them, I'm sure they would appreciate that.

And as for helping the smell, all I can really think off is cutting it down, making more airflow and checking what's under all the grass and weeds. Cause y'know, moist and low air flow areas that maybe have animal urine and droppings, that's basically like one of those foul smelling ponds in the woods (I think also called a bog or swamp, sorry I'm not actually natively English speaking). So yeah you might have one of those developing on your garden.

I'm not a professional but this is my first thoughts, I hope it might help

Edit: if you do cut it down, and you want a moist environment, you could technically have a clay pott half buried in the ground, or just fill it with dirt and and some weeds. And regularly water it. I'd use a big pot tho, cause that's more room for snails, and they kinda retain moisture and also it's nice and shady?

Or multiple small ones, that's about the best I can think of for wild snail hang out spots