I'm glad that it helped.
About the "filter": No, the old URLs in the DB need to be replaced with the new URLs. So it is a DB manupulation whatever the method you use. If you are comfortable with dumping the DB, do a text search-and-replace and feed the dump back in to the DB, you can do just that.
Having said that, a https://docs.moodle.org/en/Filter could do the same on-the-fly as you say. To my knowledge there is no ready-made one.
Use subdomain as (readonly) repository
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