0. It almost entirely depends on your attitude about how you move forward. (Edit to be very clear: this is an assessment of what you’ve described,
not a generic platitude. I hope the points which follow will give the impression that I do not think you’re an impostor, but that it sounds like you may have room for growth which will make you feel happier.)
1. This is an excellent opportunity. Take as much as can and want to from it. My experience felt a lot like what you describe, moving from a design shop which mostly specialized in brochureware websites a little on the advanced end, to a smallish startup which had more tech talent than it probably should have. After I got far enough into processing my impostor syndrome, I decided to swim (vs sink), and became much more comfortable admitting when I was inexperienced, or ignorant of even “basic” stuff, or just felt intimidated by my preparedness for a given challenge. This had the wonderful benefit of freeing up the “syndrome” portion and allowed me to accept the due praise I did get and see where I did actually contribute value in the ways I expected or better. Then whatever “impostor” was left… I was surrounded by talented people to learn from!
2. Most software engineers are far less productive than one might expect, particularly coming from a high output background. Again in my experience: this is probably a good thing. What’s better than moving fast (regardless of whether you break things)? Moving slow with clear intent, clear understanding of the value of your work, and the ability to sustain it. In other words, you’ll achieve more by being methodically circumspect, and you’ll be less prone to burnout by having realistic expectations.
3. While point the previous might suggest some degree of comparing yourself to others, please resist that urge or at least temper it with permission to be imperfect and frame it as a tool to determine how and what you could improve on. Otherwise you’ll always come up short, especially going in with self doubt.
4. Your self-expectations are different from your employer’s, and from those of your teammates. Seek feedback, often enough to know where you stand in their eyes. You might find that you’re already exceeding expectations! You’ve done this with your somewhat nondescript superior, but it’s good to get feedback from peers too. This is another good opportunity to assess whether (a) you’re working too hard, (b) your priorities are what you [or others] want them to be, or (c) whether you might at some point actually be outgrowing your role.
5. Take a vacation. It’s holiday season most everywhere. If you’re inclined, do a side project that focuses on whatever interests you but might feel a more slight challenge; or just chill because it sounds like you’re doing fine but stressed. Either way give yourself a win.