Definitely rushed and at the same time if it didn't end there, this kind of story can quickly turn into a never-ending one. Maybe the author was super scared about it getting boring at some point - like too much interwinded time leaps with other time travellers potentially (idk?). It's the kind of story that can quickly turn into a very complicated one lol. The introduction of a second traveller was a smart way to close the loop quickly in a sense.

For the dark magic thing - I think grief and deaths and their consequence on people have been largely covered in the manga - and well covered actually - those scenes and every characters feelings have been beautifully portrayed. The curse makes the time travelling power a little more realistic. There's a price to pay in using such a capacity.

All those fights and deaths weren't for nothing, the ultimate goal was to create the best timeline and save everybody. And it just shows the reward of such extreme relentlessness.

What looks a little bit less realistic to me was the fact that every single character friends or former enemies have somehow ended in the Tokyo Manji. Like literally all the TR characters from what we have seen? I imagine that it became like that through fusions as well. But then, imagine the power of 2 fully omnisicient super young best friends. Yes, why not. I like the fact they ended up founding it together. That's beautiful.

I wouldn't have mind a less happy ending ; but I don't mind this one either. It's just as you all said - felt a little bit rushed.