Factual error: A piece of ship that size crash landing into a planet and not being destroyed would be an extinction level event. The asteroid that supposedly killed the dinosaurs was a lot smaller.
Factual error: After the speeder chase the gang lands on quicksand. Not only do they not sink into it until after the stormtroopers are dealt with, but they suddenly sink at great speed. On top of that quicksand is heavier than air so there is no way that it could have floated and formed the roof of a cave like that. It would simply fall in and sit on the floor, in point of fact it would never have been able to sit like that at all.
Answer: It's a group of Sith cultists known as the Sith Eternal. They have been based on Exegol for many years.