What if SpringBoot WebFlux project will use blocking JDBC driver?

No.

C.1. How Do I Wrap a Synchronous, Blocking Call?

It is often the case that a source of information is synchronous and blocking. To deal with such sources in your Reactor applications, apply the following pattern:

Mono blockingWrapper = Mono.fromCallable(() -> { 
    return /* make a remote synchronous call */ 
});
blockingWrapper = blockingWrapper.subscribeOn(Schedulers.boundedElastic()); 

• Create a new Mono by using fromCallable.
• Return the asynchronous, blocking resource.
• Ensure each subscription happens on a dedicated single-threaded worker from Schedulers.boundedElastic().

You should use a Mono, because the source returns one value. You should use Schedulers.boundedElastic, because it creates a dedicated thread to wait for the blocking resource without impacting other non-blocking processing, while also ensuring that there is a limit to the amount of threads that can be created, and blocking tasks that can be enqueued and deferred during a spike.

Note that subscribeOn does not subscribe to the Mono. It specifies what kind of Scheduler to use when a subscribe call happens.

Yes.

You will either lead to blocking due to threadpool exhaustion, or if its unbounded, resource exhaustion.

In addition, there is no such thing as backpressure in non reactive drivers a key tenet of reactive programming.