Once you build the last building, are there more buildings available?

The last building is the Summer House, there are no more buildings after that. Your strategy from here on out is to upgrade all buildings to level 6 if you haven't already and to buy decorations with high-flower value to continue progressing. The higher the flower value, the further you can progress (highest 3 are Fire Engine, Grand Greenhouse, and Park Playtime at 2400 flowers each). If those are not yet available to you, then also look at the seasonal set for high value items.

You'll have to play old scenes repeatedly to earn the coins you need.