Do not "ssh/other forms of remote" using ip addresses. Use domain names or local domain. It is easier to remember, is more secure (if configured in DNS), and less prone to errors.

> Can you redirect ports to specific local machines using only ipv6 (that implies they keep constant addresses)?

Yes. Use domain names in configuration files. It more robust, easier to read, and is better protected against network changes on the local network.

I have been part of multiple ISP changes and searching through configuration files for ISP specific IP address ranges is never fun. It wastes time and is prone to errors. In enterprise settings domain names rarely changes and even when they do, the old primary name are usually retained for backward compatibility. An ISP can get replaced fairly quickly if an alternative is cheaper or provide a better service.

> Can you easily switch between two internet connections going through different routers that are plugged into the same switch for any machine on your local network using only ipv6?

Are you talking about BGP? BGP is a fairly complex protocol and uses some archaic configuration syntax, but even so there are generally no differences between ipv4 and ipv6. It is the same pain making sure both ipv4 and ipv6 switch between the two routes correctly.