på dansk ville man sige: Sidst vi optrådte var i april, eller bare: vi optrådte i april
Does the Danish sentence also mean: We performed some time ago in April? Is 'tilbage' usable that way?
Yep. It is like "we preformed back in April"
Is "performede" okay
As far as I know performede is included in the Danish dictionary, however, you would rarely hear people use it in everyday language.
What sense does it even make?
When does one use optradt and when optradte?
Jeg har optrådt = I have performed
Jeg optrådte = I performed