There is this thing called fountain that might peak your interest. Its kinda like markdown but specifically for screenwriters. There are many apps that work with it in different platforms. Personally i have used afterwriting but there are many apps available so look for one that you may like and works for your platform.

Since fountain is just plain text you can export it and import it in any other fountain capable app. Hope this helps!

I use scrivener and it works well. It has both Mac and windows versions that are pretty well supported.

It’s good old school on prem software as well so you can find pirate versions for both OSs and they don’t need to do anything too fancy to get working.

I’ve also saved all my projects to Google drive so I can access them across devices and it’s worked well too.

I’ve also heard of established writers using scrivener and exporting to final draft because they don’t like final draft but it has become the standard in screenwriting. You can use scrivener for more than just screenwriting too