Give the Thermolite blocks two coats of diluted PVA first, letting them dry out between coats. And then patch with bonding.
If youre going to retile the walls put the bonding on so it is flush to the finish plaster.
Thermolites suck all the moisture out of the plaster hence the reason to 'size' them first with PVA
And just tile straight on top of the bonding ?
Not be me it be a professional fitting my bathroom and tiling.
I have now come across a patch where it’s been boarded to the thermolite and skimmed.
The lad who’s doing job just said if I could remove all old tiles if would be a great help and then he would call in to asses what’s needed plastering wise.
Also pipe dripping leading to my cistern so had to remove that today and stick a bit of wonder tape on and retighten and fingers crossed. Stopped leaking.
If I was more confident I would attempt to do my bathroom myself but scared I balls it up.
New suite. New shower unit etc.