Yeah, there’s no umlaut in Bifrost. Umlauts may look very 70s - 80s heavy metal cool and it arguably worked for Motörhead, but not for anyone else, sorry. No, Mötley Crüe is not a valid counterexample.

Btw, “ö” is pronounced like the “i” in “girl”. Now pronounce the words above out loud and you’ll realize how you look to half of Europe. (Much less actually, but why let facts ruin a perfectly good argument.) Ø is pretty much the same letter and is pronounced similarly, except maybe in Danish, but Danish sounds eerily similar to goats choking on their second helping of shrooms, so there’s just no way to know which sound is meant to be “Ø”. Or any other letter, but I digress.

Bifrost is actually pronounced like “Bifröst” in several Swedish dialects, but that too is severely uncool so my point still stands. If I were to make a comparison, those dialects are about as sexy as, say, a Yorkshire, a Cockney or any US dialect. You don’t want to sound like that, now do you? Of course not.