ergo insperata tandem tellure potiti
lustramurque Iovi votisque incendimus aras,
i’m not sure about the first line. “Therefore, at last we (made/reached) land - more than we had hoped for”. The second line’s ok i think: “and we purify ourselves for Jupiter and light the altars with our offerings,”
I’d say that potior with Abl. could mean “to get possession of”. But potior with Abl. meaning “to reach” is also not unheard of.
Insperata can also mean “unexpected”, but that would not go well with tandem. I can’t imagine how an unexpect thing can finally happen.
So, at last we have reached this land that we didn’t have hopes for?