Are you monitors positioned too low ?
Sit-up straight and look at your monitors with your head perpendicular to the floor your eyeballs should be even with the middle 1/3 (about) of the screen. I my case: used to some old books to raise my monitors.
How high is your chair ?
Your femur should be level with the floor , or even knee slightly higher. When your knee starts dropping below the hip this will change the angle of your hip and pull on your back making it more difficult to sit upright.
Keyboard height should be in keeping with arms parallel to the floor.
Use a stool with no back
Tractor style seats have a nice contour to cradle your pelvis. Here is a photo of what I am using. There are other styles google "tractor seat stool"
Tired of sitting ? Take short breaks, e.g. trip for water or stand a bit while on phone.. Engage in supplemental exercises to strengthen your posterior chain.
Once you become accustomed to using your abs & posterior muscles to sit up you won't even give it a second thought.
My last office chair was "The Sitting Machine" from Herman Miller a.k.a. the Aeron chair, even this chair after a while is not comfortable. I found the mesh TOO soft which was pulling on the flesh of my thighs, quite annoying. I found relief with a stool with regard to flesh pulling on my legs.
I realize my opinion & solution may not be popular but thought I would share.
I would recommend spending HKD10k and getting an Aeron medium size chair from Herman Miller
Aeron - Office Chair - Herman Miller
Wonderful chairs, perfectly suited to HK weather with their mesh. Do absolutely sweet FA for posture/back problems, which was a label stuck to them well, well after they were first designed (Herman Miller never said anything about them being ergonomic).
Also about 7K for the base model, which is a bit out of alignment with the OP's request for a pillow.
I've been a very happy owner of an Aeron for 10 years now. And one of the best things about it is the warranty - last year the mesh on the seat pan started to come away slightly from the frame. This was one of the gradually, gradually things, just one thread at a time, but after a while it became a noticeable issue.
So, more in hope than expectation, I emailed the Hermann Miller distributor here (who had changed in the 10 years since I bought it from the previous one, and asked about the 12-year warranty). The reply was simple - sure, no problem, send us pictures and we'll get the part and fix it for you.
And so it was done a few weeks later. A new seat pan: no fuss, no charge. Well, OK we could say the charge was build into the orignial HK$7K or so, but nearly 10 years after purchase I was impressed.
Has anyone tried an Ikea LÅNGFJÄLL chair as an option for long-hours of desk sitting? I do generally prefer Aeron chairs though.
https://i.pinimg.com/originals/ff/17...2f2e9bfbf1.jpg
LÅNGFJÄLL Swivel chair - Gunnared light green, black - IKEA
https://www.youtube.com/watch?v=gCoJ50Wj0jY
https://www.youtube.com/watch?v=sGUGfaZofFw
Been using BackJoy for years, it was the only thing that solved my back and neck problems.
You have to pull it forward enough so your butt is properly 'cupped'. It's well worth the $550.