Grails 3 Runtime Automated Database Custom SQL - database

Question

We have a large application with about 100 tables. We have been updating the database manually between releases.
We also recently switched from Grails 2 to Grails 3.
We now need the Application to (for the first time in a while) create a new Database from scratch, and this is the first time since using Grails 3.
The database is varied enough that it needs some manual runtime customization.
To complicate matters, the application is also using Quartz.
The problem is:
After the Application initializes the tables, but before the Application and Quartz start running, we need to inject this set of custom SQL. Where is the correct place to insert the SQL?
I have tried some things like Application.groovy and Bootstrap.groovy, but have not been able to determine (with Grails 3) the appropriate location to inject this custom SQL. For example the Quartz tasks are running and attempting to access certain tables, but they have not been "corrected" yet so the app is throwing errors.
UPDATE
I tried the following in Bootstrap.groovy
Tag.withTransaction {
String updateSQL = "ALTER TABLE tag DROP COLUMN class;"
def sql = new groovy.sql.Sql(dataSource)
sql.executeUpdate(updateSQL)
}
Tag.withTransaction {
Tag newTag
newTag = new Tag(name: 'TAG 1').save(flush: true)
}
Tag.withTransaction {
List tags = Tag.findAll()
println("=== Tag Size = ${tags.size()}")
}
The executeUpdate() throws an exception that column class cannot be found.
However if I reorder the three sections and use the following:
Tag.withTransaction {
Tag newTag
newTag = new Tag(name: 'TAG 1').save(flush: true)
}
Tag.withTransaction {
List tags = Tag.findAll()
println("=== Tag Size = ${tags.size()}")
}
Tag.withTransaction {
String updateSQL = "ALTER TABLE tag DROP COLUMN class;"
def sql = new groovy.sql.Sql(dataSource)
sql.executeUpdate(updateSQL)
}
then the executeUpdate() completes successfully (although it is still too late as the Quartz jobs are already running).
I do not understand this at all.
Thank you for the suggestions. For now I will try the Database Migration Plugin, but would still appreciate other suggestions.