Has it been updated to the latest version yet?

The latest Daggerfall Unity version is 0.16.0 RC, and GOGDB shows GOG has 0.13.5 beta, so I guess not.

Daggerfall Unity's base game is already finished right?

Like if you play it as vanilla as possible its the 100% complete experience? All the main quests are there, the random side quests are working as intended? Mechanics are as they should be?

Yes, it is basically feature complete, and much more.

According to the roadmap only two main versions left, 0.16 and the proper 1.0 to end the game development officially. But it appears to be a couple of abstract, hypotetical bugfixing kind of versions at dev discretion

And hopefully an engine port too, but I understand if they want someone to go to the task of porting it away from the poisoned well to something like Godot.

I don't think so, honestly.

The development was a long trip of years. And the dev already said that he would be less and less involved because it is not really critical. Things here and there, the possible future development will be puntual bugfixing and basically based in the work of the other guys who have been helping in specific features, modding details etc that made the port complexion faster and fulle of new features

He also wants to be more involved in his own game, engine, prototype

Sounds good, but GOG will probably reply in 2025.

Try 2077.

He tagged GoG in the post, hopefully that produced a noticeable enough ping on GoG's radar.