Indeed.

Well, I lived in Manchester England for 2 years, and most of the people there would say "thank you very much indeed"...so, I would say that depends on the region you are

It's perfectly acceptable but probably not heard that often in conversation between friends, if that is what you mean by "informal".

You can use it but it's a bit upper class/formal/hoity-toity. You can use a thesaurus to search for synonyms, but without actually living in an English context, it'll be hard to discern what's appropriate. https://www.thesaurus.com/browse/indeed Normal: absolutely, naturally, undoubtedly, of course, agreed Informal: for sure, for real (really informal)

It depends on the context!