All above advice is good. I'd add that it may be as simple as you're not buckling them tight enough. Particularly the middle buckle. The liners start to conform/pack out after a few times so you may need to tighten em down. If the shells were fit in a shop the size should be o.k. Your foots probably moving around when you land in the back seat.
THIS. I can confirm having finally gone to a bootfitter recently and learning the process/getting everything dialed in.
I'm currently in a boot that's definitely smaller than I otherwise would have purchased on my own. You most likely have too much room in the front of the boot, so when you land backseat there's that space for your foot to slide forward and slam the toe box. I used to have this problem. I got custom footbeds with a custom liner (intuition) and now my boots are so f'ing tight there's no way my foot would move around in them.
My boots will actually be a little too tight for a bit, and that's part of the process. Once I put 50 or so days on them they're gonna pack out and fit perfectly. But if you go with the smaller size they can make adjustments to conform to your foot- in my case they had to blow out the big toe a little bit for me to create some room, but if your boots are too big they can't do anything to make them smaller per se.