[exim] relay_domains and transport files

One of my Exim boxes is used as a spam filter, mail isn't stored
locally, I have two files:
relay_domains & transport that list the domains and their destinations.
/etc/exim/relay_domains contains:
dguebert.com
gulfcoast-materials.com

/etc/exim/transport contains:
dguebert.com: mail.dguebert.com::2424
gulfcoast-materials.com: mail.gulfcoast-materials.com::2424

Then exim.conf contains:
   domainlist relay_to_domains = /etc/exim/relay_domains
 and
   internal:
     driver = manualroute
     domains = +relay_to_domains
     transport = remote_smtp
     route_data = ${lookup{$domain}partial-lsearch{/etc/exim/transport}}

This all works quite well, however, I'd like to get rid of
/etc/exim/relay_domains by modifying domainlist relay_to_domains =
/etc/exim/relay_domains in some way.

Any ideas - everything I have tried has broken this well oiled machine.

BTW: reason for change is to keep me from making stupid errors, when I
need to add or delete a domain, I must edit both files, not a big deal,
but one of these days, I will make an error (hasn't happened in 45
years....)