I think you should watch it
The first half may have a lot of plotlines that seem frivolous or melodramatic, but it provides great contrast for the actual masterpiece second half. If you're even a little entertained, watch until the end.
The story takes place in the years preceding and during the French Revolution. It's historical fiction: emphasis on fiction, a lot of the shit even with real people just didn't happen. So consider all the characters as just characters. I think the anime does a great job at development: character development and world development. Slowly over the course of 40 episodes you can see France change and the characters change with it. Oscar starts off as a noble and very loyal to the queen, but she's also a person with an extremely upstanding moral character, which, without giving too much away, doesn't lead to her staying that way.
It feels like a 70s anime for sure, but also the best 70s anime to exist.
I think Oscar may be my favorite anime character ever.
It's an amazing show and I 100% agree that even if the first half can be a bit tedious, it establishes all the characters to a degree that is crucial for all the big events that happen later in the series. I feel like at one point I was questioning if they were even going to touch on the French Revolution because the melodrama was just so on point you'd not expect them to just go through with it. From my understanding this show is responsible for a fascination with France in Japan and people expect France to be extremely different from what they actually see in reality. I also believe that Oscar went on to become the inspiration for a fairly common yuri trope of the ouji-type girl. Even if you're not particularly into the show on the merits of it just being flat out a masterpiece, if you have any interest in the cultural history of anime, you see a lot of stuff that would go on to be tropes and character types.
If I had one wish to improve it even though it's just a personal gripe
spoiler
I would've just had Rosalie and Oscar be the romantic couple rather than the guy Oscar picked in the end, though it still worked out.