When you’re in a full seat and you feel like you’re slouching think about keeping your shoulders back
Tuck your bum underneath you ! No stirrups are great
There is no secret, you need to ride without stirrups to find your balance and position. Sitting on your vag is not good because it will compress your vertebra.
You need to sit on the middle of your saddle and let your legs go down.
To keep your back straight, you can try an old method: put a whip on your back (behind your shoulders), it will force you to stay straight.
I hope i could help you :)