I feel your pain.
Went through the same same thing when I bought my first kayak seat. It was very similar to the one that came stock with the OK Caper... low back seat, very little back support and adjustment, and no padding for your butt.
One thing that might help without spending any money, would be to make sure your seat's position is correct (for you) so that you are perfectly fitted to the molded in foot wells of your OK kayak with knees slightly bent. By tightening both straps it will move you closer or by loosening the straps, will move you slightly further back, depending on what feels best to you with feet in preferred slot. By pulling on both straps at the same time while seated, make absolutely sure what support you do have is perfectly square. Once had strap on one side loosen and it had me slightly tilted to one side and though I didn't notice the difference right away, I had a back problem big time the next day. Another cheap way to get much more comfortable is to get a self inflating seat pad (cheap, adjustable during the day and much better than a solid foam seat pad in my opinion) . Lastly and the most dramatic change in comfort level would be by biting the bullet and getting the Surf to Summit Expedition seat.
I can go 10 hour days on water without getting out of kayak to stretch, easy, and have had some 14 to 18 hour days on the water without back discomfort.
PS.. the only way to get more comfortable is to get a self inflating pfd as the straps on your back don't cause problems like some of the foam filled pfds do.https://www.surftosummit.com/https://www.amazon.com/Sports-ComfortLit...+inflating+seat
A good rule of angling philosphy is not to interfere with another fisherman's ways of being happy, unless you want to be hated.
Zane Grey, Tales of Fishes, 1919https://vimeo.com/73372194