Sore seat

Some modern car seats are ridiculously firm.. what kind of car is this.

There's a range of aftermarket cushions shaped perfectly for a car seat that are worth considering. Amazon/Ebay etc have a good range locally.

An expensive fix but I fitted Recaro seats to solve the issue with my wifes back. There are some lumber support cushions available which are adjustable you could try

My first suggestion is to have your conditions correctly diagnosed. Jumping to the incorrect solution will only create more problems. You need to understand the causal relationships before getting a solution(s) that works for each of you separately.

A personal example would be when I experienced severe back pain suddenly and only after around 30" of driving. My wife already has a badly damaged lower spine. Car at the time had excellent seating.
A trip to my physio found two problems, one with my lower spine and another where the sciatic nerve joins with the hamstring. Both need on going treatment but from the seating perspective two separate fixes as well. First was to go to a specialist chair seating shop and get some memory foam cut in a specific way as required by my physio. Second solution was to make physical adjustments to the base of the seat to bring it past completely level, almost all cars seat base cants backwards, and then using the wedge to take the pressure of my lower back. That helped greatly and succeeded thanks to specialist advice on what the problem was and how it could be remedied while driving.

We changed over cars a couple of years ago and purchased one with full electric seat adjustments which has been so good we no longer need the memory foam wedges. This years trip was 14,500km through the NT, Gulf Country, Cape York and back to Adelaide towing our AOR Matrix over some pretty ordinary roads. Neither of us had a problem on any day of travel, always feeling relaxed and pain free.

Rob

Alan my wife and I both use an inflatable cushion from AirHawk - https://airhawk.com.au. These come with a simple to use pump allowing your desired inflation pressure and comfort to be achieved. A happy customer couple with no affiliation with the company.
May be a far cheaper solution than Recaro seats.
Hope this may help.
Chris

My wife had a problen sitting for any length of time. We purchased a GEL cushion from autobarn, they have two shapes and the one we chose works very well, Problem solved.
Cheers.

You could try one these memory foam cushions $19 Kmart
bought one for the 80 series for the 9000klm return trip to Cape
York last year, dose make you sit higher and harder to get in if you have a high vehicle,
did make things more comfey.

k mart memory foam cushion