Until you've seen a toughbook cranked up to 1000 nits brightness you don't know what you're talking about. You need to add brightness so that your eyes which are adjusted to bright outdoor conditions can see the screen. No filter adds brightness.
Yes they are removable, cheap ($10-15) and they work reasonably well. Putting it on you need a little practice and a clean room (no dust flying in the air). Just try it out and that might solve your problem. Of course order the correct size and watch a video how to put it on using a credit card to push air bubbles out. I always buy laptops with matte screens because I use them outside. Lenovo makes a few like that.
solutions: matte screen, filter, maybe you can find also a magnetic one, an eink external portable monitor, and finally find your angle or buy from amazon some sun cover for laptop. Best solution, work in the shade and relax in the sun
No kidding, the heat itself from the Sun is not good for the laptop. When I am on the porch, I try to seat in the sunlight, but the laptop itself is still in the shade.