Just spent a few hours trying out website monitoring services. The vast majority had either no IPv6 support or only used IPv6 where there was only an AAAA record.
Finally found exactly what I needed in https://updown.io/r/1sExw
(referral link, I get some extra credits).
Checks both IPv4 & IPv6 connectivity if DNS records present for both and includes warnings for certificate and domain expiry. EU-based. 100,000 checks for free then modest fees. No credit card etc. needed to sign up.









