Someone else will have to advise on importing your existing messages from your old email into the Gmail account. The only way I know of to do this is forwarding them, and if the archive is extensive that's just not practical. I generally do what you said, and that's keep the old account dormant, forwarding all new messages to where I want them to be, but having the archive of existing messages available for my reference when I might need to do that. It keeps my new account "cleaner" from the outset.
When you delete a message from any account and it goes into trash it will retain the same read/unread status it had at the moment of deletion. In the end, that really doesn't matter, as trash gets purged of messages that reach 30 days old regardless of their read or unread status. Outlook does allow you to change how quickly a message is marked as read once you open it (and you do have to open it), but if the intention is not to open these messages, which I'd have to presume it is, then just delete them in unread state and forget about them. Trash is trash.
Brian - Windows 10 Pro, 64-Bit, Version 20H2, Build 19042
One does not discover new lands without consenting to lose sight of the shore for a very long time.
~ André Gide