Not having a gym membership is no excuse.
-One of the exercises has you laying on the floor. Use a broomstick instead of a weight bar
-For the face pulls, you can use your bed. Hang over the edge face down and pull your arms in that direction

Stretch bands and door knobs are also useful. If you've started heading in that poor posture direction, a new chair won't fix it, all it will do is slow it down unless you get the right muscles participating again.

I grabbed one at an old job that the guy stopped using. It was actually pretty good, but the foam pads for the butt and knees wore out really quickly (weren't that good to begin with) and got pretty uncomfortable. I think most of them are pretty cheaply made and will be like that from the start.

Do lots of core exercises to strengthen your stomach and back.
Do yoga.
Remind yourself, "don't sit like a twit, sit up, back straight."

I have a keyboard that is on a tray where the center drawer of a standard desk would be. The mouse pad is also low on the right side. At one place I used a computer most of the day I made an overlay for the upper RS drawer. I couldn't stand to look at the others with the keyboard and mouse on the desk top and leaning into the screen.

Sit Back!

Keyboard on the lap is easier on me too

Just to report back, my bench is 34" and the shelf is 20".
I did some rearranging and moved the monitor out from under the shelf forward and that pushed the keyboard forward, that also allowed me to raise the monitor.
Seems to be helping.

And to reiterate I'm (my computer) at a work bench where I do repairs, the computer is just for emails, work orders, search for parts.