Very Limited Lower Back Mobility - Can't bend at the hips- Tight Psoas/Hamstrings? from cycling or sitting?

Hey Guys&Girls,

I wanted to ask for some advice online to work out the best course of action to be taken.

Bio:
I'm 28 years old, Male
Cycled to work for 5 years
Lots of sitting at computers. Slouching.
Fairly healthy (exercise 4 times a week)

There's a lot of tension in my lower back and hips. My guess is that from bad sitting habits and a limited range of motion from cycling(without ever stretching), muscles in my legs and around my hips have become tight and possibly shorter resulting in very bad hip and lower back flexibility.

When trying to bend forward at the hips they stop at about 45 degrees and I only start to bend down from about my T9? vertebra.
When sitting upright on the ground, legs straight ahead of me, my spine has a big curve at the bottom.
I've attached some pictures so you can see what I'm talking about:

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Ultimately I know that its going to take a long time and a lot of effort to undo years of bad habits.
But if anyone has any idea of what I should start to do and what course of action I should look into, I would be really grateful to hear them.

Many thanks in advance.