There are already window films that come in rolls. Some are tinted, like aftermarket car tint, and some are reflective. They can make a significant difference.

It's difficult to achieve perfection on older windows, because there's probably grit stuck on the glass like cement, that's nearly invisible but could still try and cause bubbles.

But there's probably tips all over the internet if you look. I've used sewing needles to put a teeny hole that the trapped air can escape through when the film is squeegeed, with decent effect.

And overall, you'll have a much more pleasant, even temperature across the room. And the bills go down. Overall I think it's worth it, even though perfection was out of (my) reach.