This was going to be my next headache, trying to understand what I would need in terms of a TS. Probably should have that decided really before I go for my final choice of UFH setup, but reckon I can delay my TS choice until later. Will need to have a family meeting to decide how many showers people intend to take when living in 'normal' conditions again to determine heat and water demands. I may come knocking on your door later!
Round here there is a 'bathdoctor' and I'd challenge anyone to find the repair he has made.
I've worked in a lot of properties where he has made repairs, he is very good,
As @Barney12 said, diy kits are only as good as the skill of the person applying them.
EPS wrapped in a vermin mesh and then a galv / stainless cattle grid atop, bridging the EPS and supported by both cold and warm slabs for load-bearing (driving over ) ?
Surely this is quite straightforward to resolve ? What about a thermal break in the heated screed / concrete and then only heating 8/10ths of the garage floor?