To make sure I wasn't going insane I changed the DNS to Google DNS and it works straight away, does Zen block the use of Cloudflare DNS on their network?
No they don't, I was using it for months until I recently moved to full resolution.I'd question your "bridge mode" though as AFAIK the FritzBox doesn't support bridge mode.https://en.avm.de/service/fritzbox/fritzbox-5490/knowledge-base/publication/show/3233_Can-the-FRITZ-Box-be-used-as-a-modem-bridge-mode/I don't know if this applies to the 7530 but I suspect it would.I do see this guide https://www.edpnet.be/en/support/installation-and-usage/internet/manage-fritz!box/how-do-i-configure-my-fritzbox-in-bridge-mode.html but I'm still sceptical that's foolproof when AVM specifically say they do not support this configuration.
If you are after speed, either Pi-hole or pfsense resolver will happily cache DNS records