I have it working now.  During the upgrade, build errors were stating the datasources needed to use "jdbcUrl" rather than "url". What I found is that if the main datasource (for the main Grails app) is set to use "url" and the secondary datasource for a Spring Boot subproject is set to use "jdbcUrl", then there are no build/Hikari errors and the app seems to run fine. There was also a difference in spelling between one of the datasouce names in application.yml and the external config file I was using for testing (case matters).