Probably Python. Confluence has good API support, but not sure how that works with the Cloud versions. Your strategy maybe to do the conversion on the server version, then move the spaces over to the Cloud once you are done.
I am guessing you have a month's worth of work ahead of you.
I did find this one app that might do what you want (except it does not support embedded images):
And you are in luck as it is available for the Cloud.
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