So as a developer advocate and a tinkerer with a little home lab, I am often setting up and tearing down infrastructure to test things out. I use AWS because that's what I am most familiar and I try to be super careful about not running up a crazy bill, but I am still occasionally caught off guard. Now I am thinking about testing out Infracost to help prevent unwanted and unintentional spending.
As far as a solution to consistent tagging -- if I am understanding the problem space correctly -- something like Cloud Custodian could possibly help. It's open source and you can set up auto-tagging policies as well as use Cloud Custodian to backfill tags. These policies use lambda functions to respond to certain actions (ie, spinning up an ec2 instance, etc) and auto-tag with the resource creator/owner.