Attic insulation

Op, why are you insulating between the rafters?

Unless you are living in the attic then this is a waste of time and won't work to boot because of the need to ventilate the attic space. It would appear that your thermal envelope is your attic floor and this is where you ought to be concentrating your insulation upgrade efforts.

Spot ventilation will not keep your rafters dry, you need continuous eves ventilation or condensation in the winter will be an issue.

You have chosen a poor strategy which will cost more for worse results, if you get the detailing wrong you put your roof structure at significant risk. The simpler approach is better.

Better to create raised platforms for storage and foot access and insulate on the floor. A continuous sheet of up to 30cm with two layers laid in alternate orientations is the right choice. If possible incorporate a vapour barrier below and on top to stop air migration (which carries heat and water vapour).

What you propose to do needs to be performed by an insulation expert who understands the critical detailing.