Use both JDBC and SAML2 IdPs in Apereo CAS 5.3.16 - authentication

Question

I am trying to setup Apereo CAS 5.3.16 to use a SAML2 IdP and a JDBC (PostreSQL) database IdP. We need CAS to try to authenticate against the SAML IdP first and then, if that fails, against the JDBC IdP.
Unfortunately, over the past weekend, the documentation for v 5.3.16 was removed from the Apereo website, so am now working from the markdown source documents in the codebase. I have consulted the manual extensively and read these posts - https://fawnoos.com/2017/03/22/cas51-delauthn-tutorial/ and CAS delegate authentication to Azure SAML - and can't get the app to do what we need.
CAS creates its SAML metadata, keys and obtains metadata from the SAML IdP (Okta).
The logs show the following entry:
DEBUG [org.apereo.cas.authentication.PolicyBasedAuthenticationManager] -
<Resolved and finalized authentication handlers to carry out this authentication transaction are
[[org.apereo.cas.authentication.handler.support.HttpBasedServiceCredentialsAuthenticationHandler#301ed37a,
org.apereo.cas.adaptors.jdbc.QueryDatabaseAuthenticationHandler#b48d4df,
org.apereo.cas.support.pac4j.authentication.handler.support.ClientAuthenticationHandler#6d3bc620]
Which looks right to me, except that I want the pac4j handler executed before the JDBC one. I don't know what HttpBasedServiceCredentialsAuthenticationHandler is but it is part of the CAS core in its source code, so I think it is supposed to be there.
The authentication request is going to the JDBC handler first and if that fails, is not falling through to the SAML handler. The authentication request is immediately rejected.
Here is (the relevant part of) our properties file (standalone.properties).
Can some kind soul please tell me what am I missing or doing wrong?
# --- UTS Library --- #
server.port=8080
server.ssl.enabled=false
server.use-forward-headers=true
server.session.cookie.http-only=true
server.session.tracking-modes=cookie
cas.server.name=${CAS_SERVER_NAME:}
cas.server.prefix=${cas.server.name}/cas
cas.host.name=
# Default theme name
cas.theme.defaultThemeName=ourtheme
# CAS session persistence
cas.ticket.tgt.rememberMe.enabled=true
cas.ticket.tgt.rememberMe.timeToKillInSeconds=604800
##
# CAS endpoint security
#
...
# logging settings
# Stacktrace settings, possible values: NEVER|ALWAYS|ON_TRACE_PARAM
server.error.include-stacktrace=${CAS_INCLUDE_STACKTRACE:ALWAYS}
##
# Database settings
#
database.driverClass=org.postgresql.Driver
database.url=jdbc:postgresql://${CAS_DB_HOST:127.0.0.1}:${CAS_DB_PORT:5432}/${CAS_DB_NAME:our_db}
database.dialect=org.hibernate.dialect.PostgreSQL82Dialect
database.user=${CAS_DB_USER:}
database.password=${CAS_DB_PASS:}
database.pool.initialSize=2
database.pool.minSize=2
database.pool.maxSize=12
database.pool.acquireIncrement=2
# kills persistent connections that have been idle for > 60 seconds
database.pool.maxIdleTime=60
# keys
cas.tgc.crypto.encryption.key=${CAS_TGC_ENCRYPTION_KEY:}
cas.tgc.crypto.signing.key=${CAS_TGC_SIGNING_KEY:}
cas.webflow.crypto.encryption.key=${CAS_WEBFLOW_ENCRYPTION_KEY:}
cas.webflow.crypto.signing.key=${CAS_WEBFLOW_SIGNING_KEY:}
##
# CAS Authentication Policy
#
cas.authn.policy.any.enabled=true
cas.authn.policy.any.tryAll=false
# Attribute release policy
cas.authn.attributeRepository.defaultAttributesToRelease=username,givenname,familyname,mail,[others]
# Disable default authenticators
cas.authn.accept.users=
#cas.sso.proxyAuthnEnabled=false
##
# Okta SAML IdP delegation integration
cas.authn.pac4j.saml[0].keystorePassword=our_passwd
cas.authn.pac4j.saml[0].privateKeyPassword=our_key
cas.authn.pac4j.saml[0].serviceProviderEntityId=urn:cas:saml:our.url
cas.authn.pac4j.saml[0].serviceProviderMetadataPath=/etc/cas/config/sp-metadata.xml
cas.authn.pac4j.saml[0].keystorePath=/etc/cas/config/samlKeystore.jks
cas.authn.pac4j.saml[0].identityProviderMetadataPath=https://our.okta.vanity.domain/app/our_okta_sp_id/sso/saml/metadata
##
# PostgreSQL authentication
cas.authn.jdbc.query[0].name=ourdb
cas.authn.jdbc.query[0].order=1
cas.authn.jdbc.query[0].sql=SELECT ...
cas.authn.jdbc.query[0].fieldPassword=password
cas.authn.jdbc.query[0].fieldDisabled=disabled
cas.authn.jdbc.query[0].url=${database.url}
cas.authn.jdbc.query[0].dialect=${database.dialect}
cas.authn.jdbc.query[0].user=${database.user}
cas.authn.jdbc.query[0].password=${database.password}
cas.authn.jdbc.query[0].driverClass=${database.driverClass}
cas.authn.jdbc.query[0].passwordEncoder.type=DEFAULT
cas.authn.jdbc.query[0].passwordEncoder.encodingAlgorithm=...
##
# Attributes
#
cas.authn.attributeRepository.jdbc[0].sql=SELECT ...
cas.authn.attributeRepository.jdbc[0].username=username,univid
...
cas.authn.attributeRepository.jdbc[0].singleRow=true
cas.authn.attributeRepository.jdbc[0].order=0
cas.authn.attributeRepository.jdbc[0].queryType=OR
cas.authn.attributeRepository.jdbc[0].url=${database.url}
cas.authn.attributeRepository.jdbc[0].dialect=${database.dialect}
cas.authn.attributeRepository.jdbc[0].user=${database.user}
cas.authn.attributeRepository.jdbc[0].password=${database.password}
cas.authn.attributeRepository.jdbc[0].driverClass=${database.driverClass}
# Specify whether CAS should redirect to the specified service parameter on /logout requests
cas.logout.followServiceRedirects=true
# Specify how CAS should respond and validate incoming HTTP requests
# X-Frame-Options - default setting is DENY
cas.httpWebRequest.header.xframe=true
cas.httpWebRequest.header.xframeOptions=ALLOWALL
##
# CAS PersonDirectory Principal Resolution
#
...
##
# CAS Authentication Throttling
#
...
##
# CAS Health Monitoring
#
...
##
# SAML
#
# Indicates the SAML response issuer
#cas.samlCore.issuer=sso.lib.uts.edu.au
#
# Indicates the skew allowance which controls the issue instant of the SAML response
#cas.samlCore.skewAllowance=60
#
# Indicates whether SAML ticket id generation should be saml2-compliant.
#cas.samlCore.ticketidSaml2=false
##
# CORS handling
#
...
##
# Memcached
#
...
# Monitoring
cas.monitor.memcached.daemon=false
##
# Service ticket behaviour
#
cas.ticket.st.timeToKillInSeconds=60
##
# Service registry
cas.serviceRegistry.json.location=file:/etc/cas/services
# -- / -- #
Background:
Our organisation plans to retire CAS for Okta in a phased transition. The first phase is to use Okta as an IdP for CAS, replacing a bespoke Azure AD/MSAL module. We are not keen to upgrade to CAS 6 given our CAS will be retired. The org's CAS expert has left and it's been given to me, as I'm a Java programmer and CAS is written in Java. So at least I can debug it. I am, most certainly, not a CAS expert and I find the manual vague, incomplete and lacking in concrete examples.