He would probably add some hydrated lime ..which will make it more workable rather than add breathability
We use bagged renders because we mix/apply by render pump and find the mix more consistent
You didn't ask for the cheapest solution and it comes down to what sort of job you want
You could go with a modern bagged lime render (coloured)
This will contain
Sand/Lime /Small amount of cement which will aid set and Will also be breathable
It's best if you find someone who has used the material before ..
Mono St astier or prb belle epoque would be ideal