Android SMS/MMS Conversation Migration (Reverse Ownership and Restore to Other Participant's Phone)

I have a complete SMS Backup & Restore XML archive containing approximately 15,000 messages spanning about 10 years between two Android users.

Goal:
Restore this conversation onto the OTHER participant's Android phone so that it appears as a normal conversation thread in Google Messages.

Source:

Samsung Galaxy S26+
SMS Backup & Restore XML archive
Approximately 15,000 messages
Mostly SMS
Some MMS (photos/videos)
Small amount of RCS content (can be ignored or converted to SMS if necessary)

Destination:

Samsung Galaxy S21+
Google Messages

Challenge:
The backup was created from my phone. Messages are stored from my perspective.

To restore the conversation onto the other participant's phone, message ownership must be reversed:

Messages I sent should appear as messages received by the other participant.
Messages I received should appear as messages sent by the other participant.

Requirements:

Preserve timestamps.
Preserve chronological order.
Preserve MMS attachments where possible.
Maintain a single conversation thread.
Result should appear naturally in Google Messages.
No forensic requirements; visual accuracy is the primary goal.
RCS messages do not need special handling.

Preferred Deliverables:

Modified XML archive ready for import, or
Conversion utility/script with instructions, or
Complete migration service including testing.

Please describe:

Your experience with Android SMS/MMS databases or SMS Backup & Restore archives.
How you would approach reversing message ownership.
Whether you expect any MMS limitations.
Estimated cost and turnaround time.

I am willing to provide a redacted sample archive for evaluation before proceeding.