For our migration projects, we use our own migration package that's available here:
https://github.com/Method4Ltd/Method4.UmbracoMigrator.Target
And, for the RTE Macros, we ended up writing a bunch of custom code that would find the macros in the rte string value to be migrated, loop through them, generate the equivalent block json and replace the macro with the block markup.
I don't have any examples that I can share at the moment, as it's all very client specific, but I'll make sure we add some examples on how to do it to the examples folder on our package repository asap!