AvatarNathan Richer

sounds like you should address your poor posture first. i would not recommend loading a poor structure until that gets addressed first. check out:

once you figure out how to hold a well aligned braced spine, then i’d look into bodyweight exercises to introduce some action, like hip hinging, air squats, push ups, pull ups, planks, etc.  use Becoming a Supple Leopard tests like the deep squat and internal rotation to test and retest your ability to get into position.  but i would start with your posture – if you have proper alignment of the spine when you perform movement, you may find that your flexibility will automagically increase. many times your muscles are tight because you are trying to move in while holding improper alignment and position.  so i would try there first and then go from there. 
report back your progress!