Some years ago I wrote a little tool to iterate all of an ISP's ip addresses and around 90% were using default passwords. Mostly homes, but some businesses.
If you want something very general and comprehensive, you can read this, although it is probably too involved for a basic "website": https://owasp.org/www-project-web-security-testing-guide/sta...
https://skylar.tech/create-fast-websites-from-your-home-netw...
I've selfhosted on 64Kbit/s modem then xDSL for years without a problem (apart from bots trying default passwords). If you are really afraid you'll run into DDOS attacks and whatnot, consider using a small 2-5$/mo VPS as reverse-proxy instead of CloudFlare to retain control of your infrastructure.
Since you're not hosting the site on the router itself, presumably you're forwarding port 80 from the router to the Raspberry Pi, so unless the security of the Pi ends up being broken, the router should be safe.
(Also I'd recommend using Let's Encrypt to get an automatically-renewing TLS cert so you can serve https on port 443 as well, and even redirect port 80 to it. It's not that difficult to set up, and you'll be improving the privacy and security of those who visit your site.)
ISP router (with disabled wlan) <-> firewall <- home router (with wlan)
and the firewall can then separate the network by port. WAN for the firewall is the LAN Port of my ISP router.
Is that a good setup?
See if ngrok can do what you want to do