is there a way to delete multiple messages at once on a mac, rather than deleting them one by one?

This may be helpful for you.

Delete messages and conversations in Messages on Mac - Apple Support

f10jo23 wrote:

is there a way to delete multiple messages at once on a mac, rather than deleting them one by one?

There is no bulk delete beyond the conversations as noted above...

https://support.apple.com/guide/messages/delete-messages-and-conversations-icht1035/mac