Updated my post:
My Ultimate Seat Cushion arrived and for the first time in months I don't have tailbone pain after seating for 6+ hours. My little kid also expressed he really liked it out of the blue and told me to get one for his chair too.
To counter the increased seat height and reduced max height for the chair arms, I used 2 sets of armchair pads to support my elbows. https://www.amazon.com/dp/B01M8F9IYC. Now I just need something to support my feet.
Edit: I got the Humanscale Foot Machine to support my feet. Tried cheaper ones before, and this is really really comfortable. I found myself rocking my feet several times a day, and the rocking angle is so natural, totally not like it’s pivoting through the center or the board in others.
I have these and like them https://www.amazon.com/gp/product/B01M8F9IYC/ref=od_aui_detailpages00?ie=UTF8&psc=1