Sorry to pull back up an old thread, but I am having the same issue and looking to use your suggested fix (moving to Zen when my contract ends in the summer).
That user has not been online since 14-06-2019. And as they are now with another ISP, I cannot see them posting here.
I suggest you start your own thread, as many of the issues you describe, are usually fixed by one of the moderators, after all other possibilities are eliminated.
No issues with them my end, nice flat ping graphs and no latency when gaming or watching netflix/movies etc..
If you already have BT, there's no reason they can't provide you fibre, just like BT. Phone them to double check, they're usually really quick and just give you a yes/no answer.
Good luck going through the elimination procedure with BT first though... 🙂