As promised.
130mm joists are not going to take the weight once you start hacking them about to run pipes through, you need the batten on the top to enable you to cross the joists to run the loops between them.
Running a 50mm + screed between the joists was quite common back in the day, however, it gives you a major headache with other services, both installing and access for maintenance.
Pipe size is critical to get the correct heat output, I suggest you contact the supplier who will give you a free floor plan showing the manifolds, pipe lengths, pipe sizes and the number of loops required. In other words, they will design it for you.
I would recommend you use the insulation and floor plates as per my pic, We used Roth products in the pic, one of the best in my opinion but please shop around for the best deals.
https://www.roth-uk.com/en/roth-downloads.php.
Lectrician, has post pics, look at them carefully, you must avoid air gaps as it stops the heat, some are unavoidable.
Big-all post misses the point of underfloor heating which should be designed to run at a constant low-temperature I.E. 42c give or take, any addition radiators or hot water should be zoned separately or combi plus 2 or more heating zones.
I agree with Woodsmith your initial plan would be a disaster, However, with the right screed (lightweight with insulation properties) and strengthen the joists it will work, you still have the other services problem, some of which cannot go in the same screed as underfloor heating.
Any questions please ask