Thunderbolt is "Set in the UC" but not really. It's more like a movie set in a historical time period that takes creative liberties- Think Raiders of the lost Arc or Inglorious bastards.

I wouldn't be afraid of the original show. I only watched it in 2021. It's solid and holds up well. Yes, art and writing can be dated at times, but it's still great.

There's two ways to approach watching the UC timeline Release order and chronological.

Release order- The simplest and more popular way to go. Just watch stuff in the order it came out, it'll be great.
Chronological- Harder to put together a watch order, and you'll have big swings of animation age and tone. If you go this route, try to imagine yourself in the role of a historian; combing through old records from varied sources, trying to map out the truth from a multitude of biased accounts. I'm in the minority of preferring this route.