I too adored pretty much all of Star Trek, but despite two attempts I can't finish Enterprise.

It started attractively enough, looking at the some of first steps the Federation makes into deep space and even introduced the Klingons as a newly encountered race in the first episode; it felt exciting, as you say, to see humanity's early rise above its bullshit of nation states, capitalism, and war to unite in the participation and exploration of interstellar space, a chapter of Star Trek history we only ever previous heard about second-hand. That first season was broadly compelling, generally well made & well written.

And then the Xindi turned up with their "Temporal Cold War". It seemed as if the executives watching over Enterprise were worried that without a constricted, monomyth story structure Enterprise would fall down dead right out of the gate, so we have to have a distinct and consistent antagonist, a Sauron and his army of malformed Orcs, they need to have a pathological hatred of humans & our protagonists in particular, and they need to ineffectively sabotage Enterprise like Gargamel ineffectively sabotaged the Smurfs.

Because clearly, in 21st century television, you can't just have a spaceship aimlessly travelling through space, encountering weird and wonderful alien cultures, technologies, and problems. You might end up with seven legendary seasons and four feature films like The Next Generation!

Season three totally gave up on the idea that this was a Star Trek, and it became a Star Wars. A silly, poorly written and seemingly pointless battle with the Empire Xindi, where the Millennium Falcon Enterprise overcame impossible odds to destroy the Death Star Xindi superweapon.

At that point I gave up. Did I miss anything in season four?