GTM has many pre-built variables off the shelf. You even have the option to create/ customize your own variables known as a User-Defined Variable in GTM. One of the User-Defined Variables I find very helpful is the Lookup Table variable under the Utilities section of the User-Defined Variables.
Data Studio makes creating visually appealing and interactive reports a cinch, it can also cause a headache if anyone edits the report or makes changes and accidentally breaks the report or data source. This has been a hurdle to get over when issues arise, until now.