I've tried a bunch of different seat cushions/modifications and everything I've bought that's explicitly for office chairs has sucked.
I bought a trucker seat cushion from a loves rest stop on a road trip and started using it for my office chair. It's been really nice. Only problem is that it tends to slide a little, but my cheeks are cozy af for my 8 hour work days.
i think people have the chairs and cushions covered.
instead, i recommend you also look into footrests. someone already mentioned lowering the chair to keep your legs angled properly, but that may make it uncomfortable to work if your desk is at a fixed height.
I'd just get a chair that's comfortable without any add-ons. I have a Steelcase Leap V2 at home, and a Steelcase Amia at work. I can and have sat in these for 16hrs plus in a day. Tempurpedic also has nice, thick butt cushions in their chairs at a fraction of the price.
The problem I have (and OP may have it as well) The chair might be comfortable for the first 3, 4, maybe 5 hours. But past that it's a problem. Plus what's comfortable for you ass isn't always comfortable for your back/neck.
At work we have Steelcase Leap V2 chairs and they're comfortable for 12 hour shifts plus they have been in 24/7 use for at least a decade now and only recently have been showing some issues.
If your legs are falling asleep it probably means your seat is too high. If you lower it, that should give some relief. As far as a cushion goes, I can't help too much. I had one for my last chair and it worked fine, but I ditched the chair for an Aeron and haven't had any problems at all. As far as I know, any cushion is gonna be just fine. Padding is padding.
Can't recommend this enough. I found a lightly used steelcase leap v1 on Craigslist for $120 (one owner). It was the same price as what I paid for a new EZBoy office chair from Costco, but the EZBoy started falling apart after less than a year. This Steelcase is over 15 years old and it looks and feels better than the other chair did when it was new. My back pain also went away when I got it, but I got a new mattress around the same time so it was probably a combination of the two... I do sit at my desk for 10+ hours a day, though.
There are other similar chairs (eg. Herman Miller Aeron), and while I hate to recommend reddit, I haven't found a better source for info on second hand high-end office chairs. If you're considering a new office chair and suffer from back pain (sorry OP, I know you aren't looking for a new chair), consider a higher-end used chair.
Ordered a reconditioned Aeron there this Summer for about half what it would cost new and I love it. And since it’s reconditioned that means Herman Miller and their shitty-ass CEO didn’t make anything from me.
Only thing is it had a bit of a funky smell when unwrapping, but it aired out quickly.
I’ve had good results with the cushionlab car-specific one. I know they make a regular one, but the car seat shape works great on the particular mesh chair I have for some reason. Getting out for an occasional walk helps a lot too, even 10 minutes.
Gaming chairs are scams. Overpriced and just have fancy colors on plywood. Get a solid officechair. They make their chairs sturdy and ready to have someone sit in them for 8 hours a day 5 days a week.
Mesh is another huge +1, this chair keeps you much cooler. People think they want leather or something. No, it gets too hot, and if you're sweaty at all it's sticky, and they don't last well. A solid mesh keeps you cool, supported, and your bony ass can sit comfortably for long periods of time. (Just remember to stand up every once in a while, for health)
I got a standing desk at work and when I was nervous I just started pacing around the whole office and creeping out my employees. Had to get back to sitting