TextMate 2: How to permanently default to soft tabs (spaces instead of tabs)?
TextMate 2 uses a special file called .tm_properties to control lots of settings in the app. It will use ~/.tm_properties if present for global defaults. You can also add one inside a source tree to control settings for a particular project.

The setting you're looking for is:

softTabs = true


Just add that line to ~/.tm_properties (or create it if it doesn't exist).

You can read more about this file here.