I figure some body weight exercises to help strengthen and stabilise your core muscles could help.
Look up the McGill big 3 stretches and try doing variations of the plank to see if it helps neutralise your posture over time
If your lower back muscles are tight, baby pose or knees-to-chest might help ease that off.
Your posterior chain muscles will kind of compensate for each other; if you sit a lot (I.e. at a desk job), your hamstrings will tighten over time, which can lead to sore knees and a stiff back.
Static and dynamic stretches for your hamstrings are great. I generally for for a standing stretch with one leg raised to a platform at hip level and hold / gently lean into it for 30s each side.
Glute bridge is also a good way to activate muscles that can become somewhat dormant behind a desk.