I may be wrong but "i dag är onsdag" means "today is wednesday" but it should have been "det är onsday i dag" meaning "it is wednesday today"

In Swedish, it needs to be "today, it is Wednesday = i dag är det onsdag" which is just a reordered version of "it is Wednesday today = det är onsdag i dag".

"today is Wednesday = i dag är onsdag" is not accepted because there is no subject in the Swedish sentence. In English, the word 'today' can be a time expression, an adverb meaning "during the 24 hours between yesterday and tomorrow"; and it can also be a noun meaning "the day that includes the current moment". Because of this, you can use it as a subject of a sentence "today is beautiful" or as an adverb "today we met again". In Swedish you can only use it as an adverb, it isn't a noun.

Same applies to 'i morgon', 'i kväll', 'i natt', and the like

I am not a native speaker, so someone please correct me if I'm wrong, but I think that "Det finns" means "There is/There are" (like "Es gibt" in German), whereas "Det är" means "It is" ("Es ist" in German).

Det finns means it exists, so it cannot be used here.