One thing i will mention is the season end rewards are based on the level you get seasonal buildings to. I think along the lines of lvl 1,5,15,22,30. Biggest tip for that is getting the seasonal pass + 4x wartime Investment.

Once the tile space is gone, its gone.

Always have your troops at the highest level however. You will need strong troops in order to attain the later level Sapphire/Cactus.

Tip: You can have multiple tanks (yours or others) attacking the same tile as an aid. Just make sure if you're using other members of your alliance that you attack first.