We have seatbelt and shoulder harness checks in our checklist as well. I would not abort the takeoff, and would fix it once I was at a safe altitude. If the FAA noticed, I would say, "I'll never do that again", and then file the ASAP.
Other than that I would self disclose to the captain, and would follow his suggestion.
Ethan, it's a lot more bumpy in the turboprop. . . maybe you don't need seatbelts in a jet
you can find out in a week or so