I use the very same 3 mm Mistral here in Denmark all winther, used the Miboot from Atan, the first years.
Both are sufficiently warm here in Denmark where water is not below zero, and still great barefoot feel.
Thicker and it feels wrong to me.
Just ordered the split toe boot also, to try how this works.
Only reason I haven't tried that earlier was because the Mistral can be used with a drysuit with socks, when really winther, or with a wetsuit, whatever you choose.
In winther wetsuit now so either boot can work
If colder and a drysuit the splittoe can not be used...
But also interested in hearing about alternatives.
It has to be 3 max 4 mm and straps around the instep/dorsum is a no no as useless for footstraps.