Java: Unexpected return type of a reactive function

Consider the following code:

How to Handle Future in Mono?

At the moment, I have a poor implementation of obtaining data from the Future using get, this operation is blocking, please tell me, is it possible in Project Reactor to convert Future to Mono?

Flux delayElements function from Reactor are not expect as mine

below is my test code:

Project reactor upgrade from v2 to v3?

Is there really no official project reactor upgrade guide from v2 to v3?