From what i understand, you seem to have got the ducks in a row. Awesome way to minimize the potential down.

Have you thought of using a powershell script to rape remaining steps. This way you might reduce the time to bring the DB to AG by a few more ticks

- Removing DB1 from AG

- DB1 goes offline to app

- Tack tran log backup off DB1

- DB1 goes offline

- Restoring the DB1 log to DB_2

- Renaming the DB_2 at SQL1,SQL2

- Adding the renamed DB to AG

- More steps ....

There is also the more costly way of doing things

- Introduce 2 new secondary replicas to the AG with DB1

- Failover to a one of the new replicas when time's up

- Remove the two old replicas from the AG and decommission

Hope that made sense.