Spring SAML2 Saml2WebSsoAuthenticationFilter custom AuthenticationSuccessHandler

I am using Spring Security v5.5.1 SAML2 code for SAML support. I have a working solution using the older Spring security extension, but I am "upgrading".

I have the SP initiated and IDP initiated flows working, but I cannot figure out how to configure the success handler redirect URL. It defaults to "/". I do not understand how I can access the Saml2WebSsoAuthenticationFilter and/or the SavedRequestAwareAuthenticationSuccessHandler to override the URL.

I set a default RelayState on the IDP and it does get sent with the assertions, but Spring does not appear to use it.

Also, using the older extension, I could store the SAML request in a DB and retrieve it when the response comes in since my app does not use sessions. I have not found a way to do the same here.

Here are my auth provider and relaying party registration as gleened from the docs and samples I found:

OpenSaml4AuthenticationProvider authenticationProvider = new OpenSaml4AuthenticationProvider();
authenticationProvider.setAssertionValidator( OpenSaml4AuthenticationProvider.createDefaultAssertionValidator( assertionToken -> {
                    Map<String, Object> params = new HashMap<>();
                    params.put( CLOCK_SKEW, Duration.ofMinutes(10).toMillis());

                    String recipient = assertionToken.getToken().getRelyingPartyRegistration().getAssertionConsumerServiceLocation();
                    params.put( SAML2AssertionValidationParameters.SC_VALID_RECIPIENTS, Collections.singleton(recipient));

                    String audience = assertionToken.getToken().getRelyingPartyRegistration().getAssertionConsumerServiceLocation();
                    params.put( SAML2AssertionValidationParameters.COND_VALID_AUDIENCES, Collections.singleton( "blah"));

                    return new ValidationContext( params);
                })
        );

Converter<HttpServletRequest, RelyingPartyRegistration> relyingPartyRegistrationResolver =
        new DefaultRelyingPartyRegistrationResolver( relyingPartyRegistrationRepository);

Saml2MetadataFilter filter = new Saml2MetadataFilter(
                relyingPartyRegistrationResolver,
                new OpenSamlMetadataResolver());

http
    .sessionManagement()
    .sessionCreationPolicy(SessionCreationPolicy.STATELESS)
    .and()
    .authorizeRequests()
    .antMatchers( "/saml2/**").permitAll()
    .antMatchers( "/login/**").permitAll()
    .and()
    .saml2Login( saml2 -> saml2.authenticationManager( new ProviderManager( authenticationProvider)))
    .addFilterBefore( filter, Saml2WebSsoAuthenticationFilter.class);

RelyingPartyRegistration registration = RelyingPartyRegistration
        .withRegistrationId("blah")
        .assertionConsumerServiceLocation( getAssertionRecipient( environment, "blah"))
        .signingX509Credentials( c -> c.add( credentialSp))
        .decryptionX509Credentials( c -> c.add( decryptSp))
        .assertingPartyDetails(party -> party
            .entityId("blah")
            .singleSignOnServiceLocation("https://sso.stuff/samlstuff")
            .wantAuthnRequestsSigned( false)
            .verificationX509Credentials( c -> c.add( credential))
        )
        .build();

I imagine that I can do the same as before somehow, but for all the docs that are supplied, it is difficult to make sense of much of it.

Thank you!