Can I recover from bugs and disease in my garden
I'm a newbie and need help. Sorry for the lengthiness- I have an 8x4 raised garden. My first attempt was 2 years ago, everything was growing so fabulously and then suddenly I lost my cucumbers and squash and my tomatoes quickly followed. I believe the culprits were powdery mildew and blight, though after my experience with bugs this year they were probably a problem as well. My biggest problem was getting overly excited and trying to cram as much as I could into my space; I don't think square foot gardening is for beginners. I gave the garden a rest last year and only had herbs and strawberries. I added fresh dirt to bring the level up and I planted fewer but similar plants this year, pretty much in the same places they were 2 years ago. Things were going well and then my cucumbers crashed and now my squash are also failing. I found cucumber beetles, but not a ton. I did not see the powder mildew on the cucumbers. My attempts to cut off any diseased vines were futile and I eventually took them all out. I started noticing powdery mildew on my squash and treated with Espoma fungicide leftover from past years, by the next day my squash looked terrible! My internet research lead me to identify the squash bug militia. I've cut out all dying leaves and killed every squash bug I could find. I've also tied the remaining squash up to allow better air flow and easier inspection, I'm not real hopeful, but trying it anyways. My tomatoes are showing signs similar to the blight I saw before- edges of leaves starting to yellow and brown and curl a little- but the fruit look fine. I have some onions that aren't well and a Greek basil and two pepper plants that are doing very well, especially considering their backdrop.
Now, my questions:
1)Aside from better spacing and airflow and treating sooner/pretreating is there any I can do or should have done differently?
2) Is my garden shot unless I relocate it? Are the issues I'm having (bugs, powdery mildew, blight, anything else that I might have misidentified) "in the dirt" so to speak? Did I sufficiently rotate crops?
3) Is there anything I can plant now, or closer to fall if needed, where the cucumbers and squash were that won't be (as) susceptible to all my current issues?
Thank you for any help!