the "double past tense" "verb + たんだった" is used for when you (suddenly) recall having done something (or someone else having done something)

"昨日買ったプリンがないと思ったら、もう食べちゃったんだった（てへ）"

but yeah I think it's done purely for wordplay in this case.

"... when you have (suddenly) recalled having done something ..."の方がいい説明かな...

Sorry for the noobie question, but how would you formally thank someone like an artist (musical, film, etc.) for their work (after having experienced it)?

Something akin to, "Thank you so much for your work on X."

@LucasTizma I'm not sure but maybe one of these eow.alc.co.jp/search?q=thank+you

seems like many follow the pattern 〜に対しお礼申し上げ

I assume I'd precede that with "your work/movie/whatever"?

I'm really not sure about the level of politeness required here and what you're thanking for depends on if you're thanking just because you enjoyed it or because you work together on a larger project that the artist contributed to

Yeah definitely not the latter. The former. But I'll be sharing it socially, so I want to be appropriately respectful in a public context.