Is it blocks or rammed earth-type construction? Either way I would assume it’s similar principles to a cob house here – tons of stuff online if you google that.
The main thing is to make sure it can breathe – if the render is cement-based, that might be why it’s coming off as moisture has been trapped behind. Use lime-based products e.g. render/mortar/paint etc both inside and out, keep the top and bottom of the wall dry and it should be fine.
If you wanted to add insulation then it would need to be breathable too, so no PU or PS based boards – sheeps wool etc would be better. With 450mm thick walls though there’s probably no need – try and draught-proof instead around windows etc and insulate in the roof. I’ve seen plenty of cob/earth houses with gas central heating – it’s not a problem. The thermal mass of the walls (and often, floor) is really useful.
Like any wall, if a structural crack appears then you need to look at why it’s moving but if it’s just cosmetic then don’t worry – you can just use a bit more loam to fill the gap, like polyfilla. If you wanted to extend you wouldn’t necessarily have to use the same construction method but if you do, it’s likely to be very slow and labour intensive. Would depend on the local availability of materials and skills.
