I've spent two decent-length cross-country round trips with a combination of camping and car-sleeping in my civic coupe, so I can't really not comment.
1. The main thing that bugs me about sleeping in the civic is the blatant advertising of yourself as vulnerable (even in spite of sticking to unpopulated areas and only when it was too late to be worth tenting). I slept in my reclined driver's seat, but I can't see half in the trunk being much better unless you really make some modifications to give yourself privacy.
2. You will be waking up when it gets light and any segment of the population starts to function.
3. In case it could make your sleeping arrangement flatter - it's not too much work to pull out the rear bottom cushion. Although this just might make the trunk threshold more of a bump on your back.
4. The first trip, I planned to do a bit of coding. I did do some, but it was quite hard to set aside the transient mindset and concentrate on something as abstract as coding. If you are indeed able to 'focus completely on your work', it's because you will basically be living at whatever coworking space you choose, and retiring to your car to avoid pissing them off.
5. The day after car-sleeping, I generally prioritized getting to a proper campsite while it was still light out. Even just setting up a tent and cooking a simple meal provides more stability and comfort than sleeping in the car does.
6. If your goal is really to do both, then do both. Consider even starting off your project with some hermit time in the woods instead of SV (get a second car battery (used) to completely run down with a car->laptop PS). But if your priority is to pour yourself into your startup idea, then have a solid backup plan for a more stable home base in case the car thing isn't working out.