Here's a script to setup db mail, copied from https://mathaywardhill.com/2017/03/01/setting-up-database-mail-to-use-my-gmail-account/
I haven't used it yet, but I do this about once a month and I didn't want to lose the script.
I don't see a flag in the script to 'set as default', I'll have to look for that when I use it first.