How do I use Mac Mail as a searchable archive of past emails and delete emails on Gmail?
You could create local folders inside the Mail App and transfer the mails from your inbox into these folders. To do so, open the Mail App and select inbox (at the top menu bar), then navigate to new inbox. You will get a new pop-up window asking you where to create the new inbox/folder (here you want to make sure it is set to local). You can also set a name, e.g. for your use case 2019 for all emails from 2019.