It took me a few rounds to realize that the plants have legs and that the scientist who turned into a cokroach was just sitting below a poster xD (hellppp me, cokroentist!). I was a bit surprised you did all the work alone,  from programming to making the asset, without using any free assets out there. You did a great job!

And here are what I found:

1. Maybe you forgot to add colliders (idk) to the plants and bullets, as they can pass through objects like the table.

2. I don't really understand the time. It opens a portal and starts a new stage. But when I tried not to jump into the portal and leave it at zero, it still decreased to -1, -2, and so on.

3. Player can  missused the turbo mode, I can even kill 100++ plants in a stage.

However, this is a nice game!