Spring Reactor and Vavr Either - spring-webflux

Question

We are trying to combine reactive programming with Either from Vavr.
The basic idea is to have a controller that returns a Mono but the input has to go through a bunch of filters which should return an Either with the reason for the filter rejecting it on left and a possibly augmented input on right.
Next are a few API calls which again should return an Either with the error response on left and the success response on right so we can act accordingly.
We are currently struggling to get this together but there is too much unwrapping in each map.
The ugly fallback solution is to throw custom errors instead so we could catch them in the onErrorResume.
I currently have something like this just for the filter part where are the filters are looped over in the doFilter which returns one Either for all of them, but then it gets tricky with the API calls as this currently only return the happy path and catches exceptions.
public Mono<OutputType> runPipeline(InputType input) {
return Mono.just(input)
.flatMap(
o -> doFilter(o)
.map(this::getAPIResponse)
.getOrElse(getFallback(input)))
.onErrorResume(err -> {
return getFallback(input);
});
}
private Either<String, InputType> doFilter(InputType input) {
for (Filter next : filters) {
var result = next.filter(input);
if (!result.equals(SUCCESS)) {
return Either.left("A filter denied processing");
}
}
return Either.right(input);
}
Maybe we are just looking at it wrong, so any alternative solution would be welcome.