I'm totally caught up on the OP manga. I'm at the end of Dressrosa in the anime.

If you know enough to get those Gintama references, you should be good to go. The first like 40ish episodes (up until the first movie) it's pretty much a Seinfeld show-about-nothing type of thing, but it sets you up with many of the huge cast of reoccurring characters and sets the baseline for all to come. By the last few seasons though you won't recognize anyone as you have so much backstory to everything going on and their mundane adventures of trying to afford their next meal have scaled up so slowly to a space epic. You really grow with them all so much along the way, and I feel the world is even more fleshed out than One Piece if you can imagine. If you know some basics of Japanese history in regard to them losing their isolation with outside countries and some basic shogun/samurai/shinsengumi history you will get some of the deeper and IRL cultural references as well. Some 10 minute Youtube videos will give you more than enough. Some characters are historical figures, parodies of course, but knowing that they were real people will make it funnier as well. I wouldn't call this historical fiction at all, but a very light alternate history wouldn't be inaccurate.

I watched season 1 of JJK, liked it enough to pick up the manga, but dropped both. I'm in my 40's now, so it has to be a standout shonen to get me into it at this point. It just felt like it went more teen than adult to me as it went on. I dropped it for Chainsaw Man and I stick by that decision.

That reminded me, I was going to say Steins' Gate is seinen so it's directed more to adults, and that reminded me of another awesome and underrated show: Golden Kamuy! It follows and Ainu girl (aboriginal Japanese) and an ex-soldier on a treasure hunt where the map is tattooed on vicious killer escaped prisoners! It is very much historical fiction, but full of gags and comedy, but you learn a lot about the Ainu people which I though was outstanding stuff I had never heard of. The story was awesome too. Lots of alliance shifting and backstabbing and plots, and nice brutal combat scenes and some great villains! If you want something more adult but still with juvenile humor (nudity jokes, poop jokes, penis jokes) this is what you want. English dub is great too IMO, that's the one I watched. It was the first manga I read because I couldnt wait to find out what happened next. Amazing storytelling and historical research. If you read the manga, each ends with notes going over the real history of the people and places in that chapter. It even earned a museum tour near the end of its run!