I think ideally you need 2-3 people attacking the teleporter constantly, and drone strikes from the cacti helping too. Then the other team members fighting very closely to the garden and maybe 1 team member sniping.
But overall with any given two teams of equal skill, the zombies have the upper hand slightly I think, especially in terms of the coins per minute. You could say the plants should do better, but then they're only doing better because of the skill level. For two teams with equal skill the coins per minute should be the same, but it's in favour of the zombies. It seems for the plants to get their fair share of the coins you need a team whose skill is much higher than the zombies.
I think the rate at which the zombies capture the garden should be reduced about 20%, or make it so the zombies need 2 more in the zone to start capturing. Or what could be done is the range at which the plants are considered in the garden could be higher than the zombies range - that way the plants don't have to be in such a tight area where they can just be killed too easily.
Also, if there is 1 or more zombies in the garden, if you as a plant are in the garden you should get +20 coins per few seconds for defending it. That gives a bit more coin to the plant and deservedly I think.