Most websites (e.g., this) seem to explain てしまう mean 完了 or 後悔. So it can be used for something positive as well, and if end up has the connotation of regrettable results, it may not fit.
is natural. This may be a surprise, you know what, I invented a new recipe!. It could be also more simply perfect, implying finally...
For the first sentence, a more valid translation would be
This implies positive surprise, as you wrote.
(し)ちゃった as a colloquial variant can be used in the same way as well. I believe in Tonari no totoro somehere there was a phrase トトロに会っちゃった.
Similarly 発明しちゃいました・しちゃった or 友達になっちゃいました・なっちゃった is ok above.