I would unlock Rook first and tell your team in voice that is the only defender you have, they'll usually give it to you. All you really have to do as Rook is drop your armor plates at the start of every round. After that watch what people reinforce and use your two panels to do the same areas or get one of them to point out spots that need it. If you do go Rook I suggest the MP5 with a vertical grip and the optics of your choice. Having much more success with that than the P90.
For attackers I'd probably go with Sledge first. Decent utility but few "must-have" situations like there are with Thermite, Thatcher, or Ash.
I agree, Rook is still my go to defender, even after playing this for 40 hrs. Just make sure that you drop your armor plates somewhere obvious, and even repeatedly "spot" the plates (left d pad while looking at them) to further alert your teammates.
I have my highest k/d ratio AND win ratio while using Rook.
Thatcher is a great attacking choice. Hardly anyone picks him, so he should usually be available. He is an under appreciated operator. His EMP grenade can really cripple a well fortified defense. Couple that with a skilled approach to window breaching (master the art of the upside down rappel, it usually catches defenders by surprise inside windows) and you can start contributing immediately, even without getting too many kills.
Also, try different optics. I had been using the green triangle and doing OK, but when I switched to the holographic circle, I routinely started getting more kills. It can really impact your game,
Have fun!