Pest control questions - zone 7b, North GA

It's my first year gardening here in north Georgia, and I realized today that I am in way over my head. Previously, I would reactively treat pest issues as they arose instead of taking preventative measures.

I've already lost all my brassicas to flea beetles - they were so quickly eaten, and then it got really hot, that I pulled those up and will try to plant again for a fall crop.

My tomatoes were being decimated by tobacco hornworm which I'd never dealt with before. I was picking them off and also treating with neem oil but that didn't seem to deter them too much, finally sprayed with Bt and that seems to be working so now I just need to stay on top of it. Half of my tomato plants have been thoroughly pruned but it looks like we'll still have some tomatoes. They've started munching on the potatoes and peppers too, so I guess I'll be spraying those too.

My summer squash is rotting at the base and covered with powdery mildew which spread seemingly overnight. I'm going to try the milk spray today but otherwise I think those are toast as well.

And finally, I just pulled out ALL of our bean crop because they were absolutely infested with mealy bugs. I didn't spray those with anything because I've never had an issue with them before and I didn't realize I needed to.

So here are my questions:

What can I do to prevent these issues? Should I be routinely spraying with neem, Bt, or whatever else before I see any problems? I don't want to over-treat my plants & potentially hurt them, not to mention the cost of these treatments.

What other pests should I be looking for? I've got corn, sweet potatoes, and sunflowers that seem to be doing OK, for now.

I would really like to keep using organic control methods but after today I'm ready to nuke everything with Sevin :(