One of the most important lessons I've learned as an engineering manager is to mostly stay away from job titles for developers. There are not many things that sow discord than calling someone Senior when someone else considers themselves better (unless you have a very objective way of defining it).

Too many egos; people who think senior means 3 years of experience; and the idea that there is a clear boundary just make the whole thing hard.

Reward people with a salary that reflects their value to the team, give them feedback, encouragement and challenge.