Yes. Lots. Two spin-offs in The Savage Empire and Martian Dreams, Ultima VII: The Black Gate and its expansion pack Forge of Virtue, Ultima VII Part Two: Serpent Isle and its expansion pack The Silver Seed, Ultima VIII: Pagan and Ultima IX: Ascension.
What about the Underground series? Worth picking up ?
Watched a few videos of Ultima 1 - 3 gameplay in action, and i have to say i'm not sure... Is it backed up by enough good writing to warrant a playtrough? I know from the first 10 minutes of IV that the later installments appear to have great writing. what about the first few?
As much as I've had fun with the first three and as much as I think Ultima III is a genuinely good game, I have to say that the series really starts with Ultima IV. The first three have no real story to speak of other than the small pieces of background information in their manuals. They aren't set in the continent of Britannia that is largely consistent from Ultima IV onwards, they don't feature a conversation system, they don't feature recruitable party members, they don't deal with the Avatar, they don't have anything to do with the virtues. Each one is basically a "kill the big boss" type game and they all feel very stand alone in comparison to the tightly knit story and lore of Ultima IV onwards.
Worth getting for historical reasons or out of curiosity? Definitely! The third one is even a good game in my point of view. But if you're only interested in ploughing through the main parts of the series and given that you've already started (or even completed) Ultima IV, I can't say I recommend them to you. The big fear here is that the earlier entries might put people off the whole series as there is little story, little lore and hardly any NPC characterisation at all in the first three games.
I think most people would do better to start playing through the series with Ultima IV. The first three games can be purchased or played later on. That way the earlier games won't put people off the whole series and they can be experienced after the real quality of the series (IV, V, VI and VII) has made its mark on them.
As for the Ultima Underworld games that you mentioned, they are definitely worth picking up! If you continue to play through the series chronologically, the first Ultima Underworld game comes in just before Ultima VII: The Black Gate while the second Ultima Underworld game comes in before Ultima VII Part Two: Serpent Isle.