I think that’s the main takeaway. Everyone’s bodies are so different that OP just has to sit in a lot of seats. And the thing is that sitting as an actively driving driver is diff (IMHO) than sitting when completed relaxed (as a passenger) and sitting in a seat for a few mins is diff than sitting in one for an hr.
OP, when you decide on what car/seat might work well for you, it might be worth seeing if you can rent the car for a weekend to really test it out?
As someone who doesn’t have back issues (yet) but has difficulty finding comfort in seats, My favorite seats that aren’t from an ultra-luxury car are:
BMW X3 Sport Seats (Standard seats but would prefer if it was a little softer)
Acura RDX
Volkswagen ID.4 Leatherette Seats w/ “Massaging Lumbar Function”
Tesla Model Y (soft but needs more thigh and shoulder support)
Seats in the new 7 series felt pretty amazing.
Well said, I feel very comfortable seating in BMW comfort seats both as a driver and a passenger.
Even my wife’s X1 sport seats feel decent but the pain quickly comes back as soon as I start driving (operating the pedals).
I don’t have back pain but I am overly fussy when it comes to seat comfort. The short test drives on smooth tarmac are never enough so I always try to either rent (on Turo) or see if I can find the model I’m interested in at a local Carmax as they do 24hr test drives which I’ve always found useful.
I have to say one of the most comfy seats I’ve ever had was a Highlander. I’m all about squishy thigh extenders.
I personally find lexus seats are comfortable than most of other cars that I have. Lexus does not have a lot of driving thrill when compared to german cars but it is designed with a lot of comfort. Maybe give a try to the LS500 and LX600 at the dealership. I used to own LS460 and LX570 and both of them have the most comfortable seats among cars that I owned thru my life. I think for new generation LS500 and LX600 they make the seats a little bit harder than last generation but still they are comfortable enough.
Oh, I had forgotten about those! I’ve heard (or read?) that BMW comfort seats are supposed to be amazing. My only experience w/ BMW seats were the standard ones in a 3-series from several generations ago. They do not rank among my favorite seats… 
and 
Also, seats by themselves can only do so much. I imagine going over a pothole in a car with a stiff suspension and low profile tires will be jolting regardless of the seats.
It depends what triggers your sciatica. For me it’s hamstrings. I sat in a EQS sedan and the flat seating, legs almost fully outstretched, immediately flared mine.
Yeah I suffered from Sciatica, last 5 years, was driving a 2017 E300 as soon as my lease finished I switch to the 21 Audi SQ5, I dont have the confort seats but the sport seats are hard so it actually works , I wont be getting a sedan unfortunately cause of this pain
I don’t have a specific trigger other than sitting. Even my cushy sofa can be a problem whereas my wife’s reading recliner is pretty good for it. My office chair is a stupid $ ergonomic 2.5million-way adjustable chair so sitting in it doesn’t bother me. Otherwise, sitting is a literal pain in the ass (and leg).
When I had my Taco last year, zero back problems, maybe sitting “higher” helped in my case. Every time I get into my wife Stelvio, 20 mins and I’m screaming in pain (regular leather set, with lumbar support but no much on the “side”). Actually lease end coming up, so seats are going to be the deciding factor here as I often end up driving her car when we’re together with the kid. Q3 before that same story, couldn’t drive more than 20/30 mins.
Last week on a work trip I ended up getting a Highlander without leather seats, same story, so much pain. Thank god as that was high on the list.
Settled on a GC, leather seats are very comfortable for me, so that’s it.
P.S. my “bucket” seats in my Giulia QV are just PERFECT, low, I feel “trapped” and my body doesn’t move…zero pain… Never letting her go 
I took my sofa cushions to a local reupholster, had them replaced with the firmest memory foam they have, huge improvement.
Yes, and there are dozens of places online to buy cut to order foam in all kinds of different thicknesses as well.
Back stuff is really to the individual. I have bilateral L5 spondylitis and grade 1 spondylolisthesis and I am fine as long as the seat has lumbar support. Luckily I don’t have any nerve pain, just muscles. Part of the reason I went for an M4 without the comp package was that you lose lumbar support with the comp seats. But in the German SUV world, Porsche’s 14-way seats are the right level of comfort/performance for me. I like Audis’ seats if they have the leg extension. Other Volvo and most Americans brands have sofas for seats.
Finally, a Porsche recommendation. Let’s close the thread before I let facts get in the way of what I want.
this would be my choice as well. i test drove an aviator and it was like sitting on a couch. Genesis also had a very comfortable seat and one of their tech packages adds massagers to the front drivers seat.
OP your best bet is to stop worrying about which seat is the best and head down to your local cripple store (medical supply) and look for a lumbar chair support of some kind. or find a local wealthy cripple store (relax the back) and get something like this:
https://relaxtheback.com/products/mccartys-sacro-ease-car-back-support
or this
or these:
A good chiropractor should help.
I also heard good things about the old S Class Coupe multicontour seats. Matt Farah wrote a 3000 word editorial about them.
I must have gone to a bad one then as $1k later no difference.