Hello and welcome.
Each internal and external drive connected to your Mac has its own Trash. If you "move" something to the Trash on an external, it is not copied to your internal drive. File aren't actually moved to the Trash...Finder just sets a flag on the file to identify that it is in the Trash and will be deleted when the Trash is emptied. If you open the Trash, it lists all files that have been moved to the Trash on all currently connected drives, but the files still reside on the drive they were originally on.
You can verify this with an external drive. Make sure the Trash is empty, move a file to the Trash on the external. The Trash will show that is has files in it. Disconnect the external drive without emptying the Trash, and the Trash will then show as empty. Reconnect the drive, and the Trash will again show it contains files.
launchd, backupd, and other tasks in Activity monitor are simply logging processes and data throughput. Data written/read aren't broken down by drive - it is just computing the totals of data that they have processed.
An M2 should be lightning quick. Of course, it will take some time to index files on all connected drives. A 3 TB external could take quite a while for Spotlight to complete its initial indexing, and if files are being moved, deleted, etc. during the initial indexing, that will add time.
As far as the slowness, did you install any antivirus or third-party utilites that have any sort of real-time monitoring? If so, those have been known to affect performance and cause slowness. You can always disable those features and run them manually to see if that speeds things up.
Hope that helps!
C