As someone who ditched Spring Boot I recommend Ktor. Spring Boot has a lot of legacy design which hurts maintainability more than the benefit. It's also nearly impossible to figure out due to the entangled dependency injection, and having multiple different way to configure the same thing doesn't help.
Ktor is on the other hand much more unopinionated, and allowed me to focus on actual code not a million lines of configuration and gluing. It does not come with a lot of batteries though, database-persisted sessions etc have to be implemented on your own, but it's not much work if you are already familiar with one of the database libs (I use jOOQ, don't recommend JPA because it's an ultra leaky abstraction and a pain in the ass to deal with when you want to write direct SQL).