I think for etcupdate etc, you should be able to get that info from /usr/src/UPDATING

For port upgrades, I dunno if it applies to CURRENT, but pkg seems to work?

I usually read the release notes for each release, which keeps me up to date enough.

I dunno about your session question. In X11, you could drop those in .xsession, but Wayland is a just a protocol, so who knows.