If you don't know how to start an answer, just say 'Păi...' It buys you 2 seconds of thinking time and makes you sound like a native speaker immediately.
No, 'dar' must connect two clauses or ideas. It cannot stand at the end.
'Dar' is neutral and common. 'Însă' is more formal and flexible in placement.
Technically, 'deci' is a coordinating conjunction that should connect two clauses. However, in spoken Romanian, it is universally accepted as a discourse marker to start a thought.
'În fine' is informal/neutral and used to drop a subject. 'În concluzie' is formal and used to summarize a logical argument.
