For facts sake:
Foam will not wick water near as much as gypsum based adhesive
That does not mean you can foam a board on and problem solved
I think @Dollar is on the money (no pun intent)
Battening an uneven wall is not much harder than battening an even wall just a case of packing it out where needed which is extra faff but not really extra work - it can be quite satisfying?
If render on outside has sbr in it then water can’t escape externally and if you batten and DPM/VCL inside the water can’t go inside wards either your wall would become like a massive tampon and when they are full they leak somewhere.