That is correct, monitoring data cannot be migrated. If you need the old data, you can always either keep it in your managed instance for a bit or export it via API and keep it somewhere else - but your new SaaS cluster is not a good palce where to put it. The most relevant reasons as to why not are that you can only import data 1 hour into the past at maximum (so the imported data from your managed instance would not look correct) and that you can't re-create the entities of your old environment, like services or process group instances - even if they are now monitored in your new environment, you would need to somehow map them (as you only get the ID), which is not, by any means, an easy task. Also, as per the last point, you can't export and import monitored entities from Managed to SaaS.
All in all, your statement is a good summary of what you do in a migration from Managed to SaaS - recreate the configuration (usually by API is the best and fastest way) and re-direct OneAgents, as to not have to re-install them either.
