It'll heat up quickly whether the op uses EPS or PIR. Occasional use means your insulation cost gives far less return, justifying less spend.
I've seen nothing in the pics that would in any way rule out in-situ felting. One of the plusses of this crude but effectve method is it can cover all sorts of damage & defects effectively, it moulds to the existing structure, unlike rolls of ready made felt.
it's good, it's just too much for a beginner imho
As long as it's not at risk of collapse any year soon, in-situ felting would give it plenty more life. Low pitch is not an issue with in-situ felt, assuming you don't do it really stupidly, forming water traps - I have seen that done.
To be clear, using ready made roll felt would not be a good choice.