You can point a name at your home IP just as easily as any other IP.

But you have to pay and involve a 3rd party.

Are there ISPs that give you free static ips? Maybe for ipv6?

Probably not these days for v4

The validity is just 6 days, so I'd assume it's not for long lived use cases? Or am I misunderstanding something

they're only for publicly accessible IP addresses, so they'd work the same as regular letsencrypt certs - get a new one when the old one expires.

Certificate validity has no bearing on availability. It just keeps revocation lists short.