I have a similar setup with 3 different heating elements, each with it’s own different brand controller (who knows what the previous owner was thinking!).

I swapped one out a few years ago for a Heltun which seemed to meet my requirements and is designed for exactly that. It is the older discontinued model running on 500 series z-wave, their newer ones run on 700 series.

I had initially planned to replace all 3 with the same model assuming it was good - I haven’t (as yet) - it was a pretty new product and it was a bit buggy which made me hesitant. I have wood flooring on top of the concrete slab/ electric blanket in two areas and it’s advised to not heat the wood up too high as it causes shrinkage, unlike under ceramic tile, and sometimes it would not respect the max floor temperature setting. I went back and forth with their support about it and they were very helpful, giving me beta firmware to flash etc and I worked around it.

For the last couple of years it’s been mostly stable and behaving. For whatever reason, it no longer lets me directly set the target temperature in Home Assistant which it previously did - I may need to dig into that as it would be handy sometimes, but I think it meets all of your other requirements, like on device manual control, no cloud, air temp sensor and floor sensor connection etc.

There aren’t that many products out there that fit the bill, I was particularly struggling with ones that would let me set the max floor temp sensor low enough when I was comparing the spec sheets a few years back.

That’s annoying! I’m using Graphene and I just installed KDE Connect from F-Droid to test, which didn’t trigger, however it did bounce me for using Heliboard. Changing to default keyboard and reloading worked, ie it can only see my currently active one.

Using Shelter to set up a second profile, or the new Private Space feature on 15 may help provide isolation.

Halifax/ Bank of Scotland/ Lloyds does an integrity check that rejects Graphene or LineageOS phones completely.