How people feel using different car seats does seem to be a very individual thing. Your size and weight suggest that on a physical basis there should, in theory, be a good match between you and the Model 3 seat i.e a high percentage of people of your size would be predicted to find a comfortable position. Out of the hundreds of people on this forum you will get responses from some others who have had a problem with finding a comfortable position but in general people are not having notable issues. There's a possibility it's "just the seat" but I would be surprised.
(I'll preface the next bit by saying that I trained as an Occupational Therapist many moons ago. Clearly I don't have a clue about you so it can be totally discounted if it has no bearing on your own experience! ... but even if it's not relevant to you perhaps someone else will read it and think ... hmm, maybe that applies to me.)
I'm sure you will have been aware, if struggling with lower back pain/sciatica, that it is prone to go through periods where it is much worse than at others. Often it is hard to identify a trigger. Sometimes specific exercise routines or seating positions ease the problem and sometimes they seem to have little effect. We try to make patterns and connections with our environment to help identify the causes of problems and it's possible (only possible) that the back/sciatica connection with the M3 seat is not the trigger, even though it's when sitting in the car that you are most aware of it. If you have discounted all the obvious opportunities to test out the physical variables of the seat (as I'm sure you have) I'm going to throw in another angle. There are many papers published around the psychology of pain generally and lower back pain particularly. Having experienced LBP myself I know that this connection is very real.
So if a psychological component is to be part of the picture what has changed recently? Well, most specifically you have recently committed to a new and expensive car ... even more dramatically it's an EV ... was this the right decision ... does it chime comfortably with the views of friends, family, partners? Are you confident that this "range issue" isn't going to come back to bite you? Perhaps not ... but there may be other significant work, health, personal issues or whatever [I appreciate that none of this may apply to you ... but you get the idea.] The reality is that whatever the cause you are experiencing discomfort and pain. There's usually no magic bullet when it comes to LBP but light exercise, relaxation techniques and time are all likely to be part of the picture. What do you usually do to ease/resolve a flare up?
Or ... of course ... you could get rid of the car ... which may solve things in an instant ... or not!
[The first idiot who erroneously distils this down to me saying "Tesla is perfect, it's all in your head" can just <expletive> off.]
