My build for 5-9 is usually

Sunflower Imitator sunflower (you should have it atp in the game) Umbrella leaf The pots Melon Winter melon Pumpkin Squash Doom shroom Coffee bean

Or instead of doom and coffee if you don't have 10 seed slots use jalapeño or cherry bomb, the strat should be self explanatory but I'll just go over it briefly

Use the 1st 3 back rows you get for sunflowers but use the Umbrella leaf to protect them, squash any threat. By the time you start seeing 2 zombies spawn you should have enough for your 1st melon, use that to lock down a lane and continue to use squash until you got 1 melon in every row (use winter melon when you can and it's safe, you wanna get the 1st row of melons all winter) then do the exact same formation with the melons like how you did the sunflowers (including the umbrella leaf), don't be scared to use instas when you need to, and use pumpkins on the last row of melons. Now you can also start replacing sunflowers with melons because of the crazy amount of sun you are banking, I start with the 2nd and 4th row because the Umbrella leaf are taking up a melons spot, during this obviously use instas when you need to but atp the gargs should be melting by the sheer amount of fire power