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Anything in the departing user’s own mailbox dies when you suspend them. Vacation responder stops, Gmail-level forwarding stops. Moving the auto-reply onto a live account is the only place it survives, so you’ve already got the right answer.
Two possible shortcuts. The recipient address map redirects mail while the user still exists, so the rename becomes optional: Apps, Google Workspace, Gmail, Default routing, change envelope recipient. It only moves mail, so you’d still want the filter and canned response on the receiving side. GAM can’t configure default routing though, so that step stays manual. Aliases and groups it does fine.
And if you keep the rename, the -DNU account is a dead and still costing you a licence. Deleting it once the data is off frees the licence and the address. Google lets you reassign a deleted address to another managed user inside 20 days, the same window you’d have to restore the account if something turns up missing.
We script ours end to end and it prints those capture options at the end of every run. Open source if it helps.
https://github.com/PaulOgier/GAMScripts/tree/main/OffBoarding%20Google%20Workspace%20Users
Paul