most anglers who have a z7.5 or z9 (top kits are the same) elasticate the top two as you described, as for J elastic I found this quite a bit thicker than say hydro or preston rubbers, I know my mate has elastic up to purple hydro in his pole, he has 14 top kits, which comes out of his top kits with no problem again he uses a daiwa bush as opposed to preston bushes that give less diameter for the laccy to flow through, so check what bushes you put in the pole and also the diameter of the laccy.

have a look at Matrix hollow elastic and the Frenzee (Vespe?) they’re nice prices and seem good. I’ve been using Vespe hollow for years now.

Before you start with the expense of changing your rubbers I would give the inside your top kit a good clean and check the internal diameter of the PTFE bush. It is surprising the amount of debris that collects inside your pole and clings onto the elastic. You do not say which colour lastix you are using but if it is up purple in colour it should fit through the PTFE with no problems. I use up to purple with a 3.8 internal in the second section of my pole a Superba XT (exactly same specs as an Airity) with absolutely no problems.