The seat pad idea works perfectly for me and a lot of other VK paddlers - puts you low in the boat, thighs are supported, no pressure points. I use a variety of things, all cheap and readily available, to make lower back supports and hip pads. Keep refining your "cushion-on-hull" seat, and you'll find your own sweet spot pretty quickly.
One thought - making sure the seat is secure in it's position (I strap mine in) helps if you end up re-entering - otherwise, it can float around and get in the way...
Are your knees being forced apart & down You mention that the sides are nice and snug; are your knees being forced apart and down by the cockpit coaming/thigh braces? I’ve found that this can trigger sciatica in a matter of minutes. It seems to be most common with bigger people in low-decked kayaks, and also with aggressive thigh braces, especially the hooked type that you see in WW boats.
Forcing your knees apart and down presses the back of the thighs against the seat, directly on the sciatic nerve, and no amount of seat customization will correct this.
I would suggest I would suggest that, before all the messing around with the seat, try moving you pegs in or out. Sometimes what seems comfortable at first, (knees tight to the deck, or legs straight out) may get very uncomfortable on a long trip. Anyway, trying different foot peg positions is a lot easier and cheaper than replacing a seat!