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For the vine borers, there is no cure to get rid of them other than to not plant squash for a few years. But in my own garden, I have a problem with vine borers and you can do things to minimize their damage.

The most common method is to check daily for their tell tale "bore holes" at the base of the plant. They will be small holes with what looks like sawdust below them. When you see the holes, you use a light on the vine to find the borer and stab it with a toothpick through the vine.

The method I use that is less labor is to spray my squash weekly with neem oil. It is a systemic organic pesticide, which means it actually gets into the plant's system and kills the borers when they bite the plant but it is very safe for people, pets and good bugs. It works well for me. Here is more information on neem oil: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

Watering deeply means to let the water run in one long, slow session once or twice a week rather than short spurts everyday. You can make or buy a rain gauge and set it out in the garden. This will help you determine how much water has fallen on the garden. Since you have sandy soil, you may want to give the plants 3-4" of water a week as the water will drain away in the soil more quickly.

Lime or gypsum are normally used to raise calcium. Lime will change the pH of the soil, use it sparingly.

It sounds like you may have damping off. This article will help:
https://www.gardeningknowhow.com/garden-how-to/seeds/what-is-damping-off.htm

Here is an article that you may find helpful: https://www.gardeningknowhow.com/edible/vegetables/zucchini/pruning-zucchini-how-to-prune-zucchini-squash.htm

While it is possible to grow them on a trellis, fence, stepladder, etc., you could also choose the compact varieties, which are better suited for smaller areas.

We like neem oil. This article will tell you more about it:
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm