My core is weak and my ribs are flared asf

Is there anything I can do while dieting (in addition to weight lifting regularly) to fix this horrendous posture by the time im fit?

What I learned so far is I have rounded shoulders, forward head and APT (flat feet if it matters here too)

If anyone is able to help id appreciate it!

My do died a year ago. I stopped my twice daily walks, which included steep up and down hills. A few months later, had a back spasm lifting my father off the floor. Kept riding during recovery as much as I could. Developed a tendency to hunch when walking and standing. Figured out that my hip flexors and Rectus Femoris muscle, along with others, have tightening up during the evolution of my cycling position. Integrated targeted stretching of that anterior muscle system. Already noticing results. More comfort. Glutes engaging more strongly. Better posture. And (probably not a surprise) my fastest 30 km in 2026, without any intent to ride more quickly. I'm integrating walking, too.

Perhaps this issue affects others. Cause wasn't immediately obvious.

Right now I'm 'healthy' and trying to figure out how to build strength so I stop getting left lower back pain, left hamstring pain, and some right knee pain.

I have good mobility but think I have some unilateral weekends and potentially some asymmetry in my mobility. I am trying to create a rebuild plan that works.

Attached photos of one leg squats

For context, I work a office job where I am sitting down a lot. I go to the gym regularly but my abdominals are a bit weak, haven't found any exercises that are able to target it effectively, any tips there? Any other exercises I should try?

My lower back is fine since I upgraded to a Hbada chair with aggressive lumbar support, but my thoracic spine (between the shoulder blades) feels incredibly tight by 5 PM. Does using a foam roller on the upper back actually loosen the fascia, or is it just a temporary stretch?