This sounds like a project a little too far as the floor needs moisture testing and then the screed levels need looking at closely to understand how deep the levelling needs to be and what primers to use and how to manipulate and build up as and if required.
The LVT will only be as good as the...
Universal Ally Ramps will do the job but will look like what they are, cheap!
Have a look at CAT profiles or possibly Gripperods Premium range, they may be a bit cheaper than Stairrods UK
Once you encapsulate the UFH as described above, do you have the TOG rating of the UFH system?
Ideally you shouldn’t overload the UFH system and it should have a TOG rating that the underlay and Laminate flooring should not exceed combined.
The best UFH underlay for Laminate and wood floors...
Porcelin/Cermic will just need levelling and fit.
Kandean Rigid Core or any other Click or Glue LVT will need:
1 coat of Ardit NA or 1200 Screed Coat
1 coat of Ardit DP-C or F-Ball F77 Liquid Dpm
1 Final screed coat of Ardit NA or F-Ball 1200 or a screed of your choice with appropriate Primer...
For best floated results, 4mm Ply the subfloor and float the Engineered over it although judging by your photo, it looks like you could float over the floorboards as they are with a GOOD quality underlay like Duralay Multitex/Timbermate XL.
If you decide to ply, it might be worth going 6mm...
If you go down the Liquid DPM option, you absolutely will need a layer of screed over it as it is impossible to apply 'flat' So you will add some height.
If you can't let in some plasterboard etc, then I read an idea on another forum where you could rub the wood back and then use a Automotive Body Filler product, sand back and decorate over that?