I got back to this a few days ago and finally got it working.
I tried many solutions from Google Home: Could not reach [test] myapp. Please try again - #49 by Zoomtronic but none of them would work.
What ended working for me in the end was to recreate the project in google (probably unnecessary) and temporarily go back to directly exposing the HA https interface on port 443 using NGINX SSL Proxy (Or NGINX Proxy Manager) and a port forward. I set this up and then created a new project on the google side and after following the steps I finally got a prompt to login through the google home app.
After this, I switched back to cloudflared and its been working fine since. I feel like there was something in the cloudflared configuration that was resulting in the timeout message, I didnt see anything obvious (checked WAF etc as mentioned above), either that, or I got lucky and it just happend to work this time around.
Since setup, i’ve had no issues, my IP has changed, I am only using cloudflare tunnel for access and HA has been restarted.