To dry line or not to dry line

Long story short, I bought a water tight house that was unfinished. It's got a 100 mil cavity with 60 mil 'pearl' insulation between the blocks. It's at first fix stage but nothing has been done to the internal walls as yet bar chasing and electrics ran.

I've read / searched this forum and others endlessly to get advice on what can be done at this stage to insulate the house as much as possible. There seems to be convincing arguments on both sides that;

- Sand and cement with plaster finish will allow the house to be more airtight, and will absorb the heat and let it back out, but the house would take longer to heat up.
- Dry lining using dab and stick of insulated plasterboards could cause mould issues and have issues with air tightness, but the house would heat up faster.

I'm at a loss as to what to do / which would be better. The house itself is a holiday home so will be mainly occupied during the summer (but will be used during the colder months too). I'll be using radiators, oil heating (mainly timed during the winter when we're not there), stove with back boiler, and I'll plumb for solar but fit the panels at a later date.

I was leaning to sand and cement with plaster finish and potentially dry line at a later stage if I felt the house was too cold but obviously would like to get it right from the get go.

I'm in the process of finding a structural engineer with experience in energy upgrades, etc. to take a look and advise but would appreciate any advise in this area!