I guess that a new sportsbook can gain trust in this ways:
1) Support: You should have a live chat active 24/7 where players can reach an operator in seconds. This would be really important because players needs to be in contact with support if there are some issues.
2) Payments: Payments should be really fast. If payment would be really fast, player will have more safe in play with your platform, because he knows that he can receive his money asap.
3) License: A site that doesn't have license, speak for themselves. A minimum of license (the curaçao's one is sufficient) gives to the user the guarantee that site will respect rules.
4) Promotion/advertising: If site can have a good visibility, this would be really important because more visibility means more trust, if the previous 3 point are respected, naturally.
From your list, I can see License + Promotion/advertising is a
must have for a new sportsbook complete against other well-established sportsbooks. Support and payments are more cherry on the top, you can slowly upscale those when more users join the house. Promotion/advertising would need a lot of money to get enough brand recognition and require an extensive study on what kind of target audience your sportsbook needs.
In the end, take a huge amount of effort/money/time to establish a new sportsbook and earn the trust of gamblers in the already oversaturated market. Unless you know what kind of misses those well-established sportsbooks don't have, gives your sportsbook an edge.