I did some digging here on the forum in order to find a solution for the certificate error and found this:
Their advice was to turn off the proxy for the n8n “A record” at Cloudflare, turn off the Docker containers for a while and then then restart them.
After doing the above, LetsEncrypt can now successfully create a cert for my domain, and I can establish a HTTPS connection directly to my domain without being proxied via Cloudflare.
Slack is still not able to verify the Request URL, but I have a feeling that it could be because their DNS cache hasn’t yet flushed out the old IP for my domain.
I’ll re-test tomorrow and see how it goes.