Use a traditional two coat plaster system, in this instance hardwall to level followed by skim coat.
Batten and plasterboard followed by skim.
Non of these will be exactly easy at present as there is a shortage of almost all gypsum plaster products. Also @SeƱor_Bear will likely need more than just one pallet of either undercoat plaster or dot and dab adhesive to do the job if that is an accurate sample of the walls.
There's 3 solutions:
Non of these will be exactly easy at present as there is a shortage of almost all gypsum plaster products. Also @SeƱor_Bear will likely need more than just one pallet of either undercoat plaster or dot and dab adhesive to do the job if that is an accurate sample of the walls.