Rockland is low hanging fruit. It can happen relatively quickly and relatively inexpensively (the necessary track work was already done years ago). Lewiston would be a massive multi-year project, similar in scope to the original Plaistow-Portland rebuild, probably costing around $70 million.
Since the Downeaster is a state-managed, state-sponsored service, that likely means the state would be expected to chip in some money, which means a bond, which means it has to go to the voters. Before that could happen, there would need to be engineering studies, environmental impact studies, etc. If they decided right now to do that and were able to fund the studies out of some existing budget, and the studies were completed within the next 12 months, you might see the bond question on the November 2022 ballot. If the bond passed (and I believe it would), theoretically construction could start in spring 2023. But that's a fast, aggressive, best-case-scenario type of schedule.
As far as I know, Rockland has always been planned as a seasonal service, likely starting and ending at the same times as Old Orchard Beach.
Pan Am Railways — Boston & Maine/Maine Central — Delaware & Hudson
Central Maine & Quebec/Montreal, Maine & Atlantic/Bangor & Aroostook
Providence & Worcester — New England — GE Locomotives