Seat and thus backrest position is more about balance of the weights in a kayak, moving the seat back will upset the balance since you (or anyone else) weighs considerably more than the boat.

Yes.

Anyway, it sounds like the seat is fairly close to the back of the cockpit already.

If you are having trouble getting your feet in and out with your butt in the seat, there are a few tactics you could consider:
1) There is some technique involved, so practice may help. Keeping yourself from 'sliding down and forward' when you pull back your foot to start exiting can help. I find that my boats with footplates or bulkhead footrests are easier to exit and enter, compared to one with regular footpegs. I found that practicing inthe back yard was less embarassing than at the beach, when my legs were half numb after paddling for a few hours.
2) Some stretching and strength exercises can help. If you can stand in front of your workbench and put your foot on the bench, you are on the way to a smooth entry and exit.
3) Folks with small cockpit boats get used to sitting on the back deck and sliding their feet in; exiting takes a bit of arm strength to reverse the move. And it takes more balance to do it afloat, compared to the 'butt first' style.
4) You could increase the cockpit length a bit - a couple of inches more at the forward end can make a huge difference. This would be an 'intermediate level' DIY project. You would need to buy a new sprayskirt, probably, unless your skirt is a 'touring' type with a lot of adjustment in the circumference.