Welcome to the forum. As you have learned gardening can be hard work. It makes you appreciate more exactly how much effort it takes to put food on the table.
You said you are from the Southeast. It would help to know your exact location and zone.
I don't know when you planted your crops and how the soil was prepared.
Sandy soil will dry out quickly so it is good to add organic matter to help hold moisture in.
Most raised beds use a combination of soil, compost and drainage material. If you have sandy soil you could probably get away with adding enough multisouce screened compost to make up half the volume of the bed. I would throw in a bag of well composted steer manure for every 100 square feet. (10x10) space. 1/4 of a bag for every 4x4 space.
Lettuce is a cool season crop and grows well early in the year but once the temperature exceeds 86 degrees it will bolt. Lettuce should be spaced 8-10 inches apart for a loose leaf lettuce. Most gardeners don't even attempt to grow anything like an iceberg lettuce or a heading lettuce. They require double the space to grow and they are not easy.
Cucumbers is a warm season crop. They like to be planted when the temperature is at least 60-70 degrees. There are different kinds of cucumbers but the ones with the best yield are parthenocarpic varieties like Diva. If you plant monoecious or gynecious cucumbers you have to plant more than one and you have to have a polinator variety to produce male flowers. Cucumbers will require bees to be polinated to make fruit and you need to have male and female flowers open at the same time. Parthenocarpic cucumbers on the other hand produce almost all female flowers that don't require polination, so you get more fruit. If pickle worms are a problem. bag your fruit. The moths usually lay their eggs at night so bag your fruit as soon as they are polinated. I make bags out of tuille. and a purse string tie. Bt can help with caterpillars before they burrow into the fruit. If you suspect your fruit have already been infested destroy the fruit down the garbage disposal to make sure the worm does not mature. If the fruit are large enough to eat, you can cut around them.
As you noticed corn needs a lot of space. Corn likes it warm. You can plant 8-10 inches at the closest but you must plant in a block for good polination. At least a 4 x 4 block, but a 10 x 4 foot block is better. Corn is a heavy feeder so you will need to give it extra nitrogen about 3 weeks in.
Plant a diverse garden with habitat to attract predators. This is includes shrubs and nectar flowers like alyssum, fennel, Queen Anne's lace, Penta, cosmos, zinnia, sunflowers, marigolds. Provide water in a shallow dish filled with pebbles. Shelter, hollow logs, bat houses, artificial beehives.
If you establish a diverse garden and have a lot of predators around, you won't have as many problems with pests. It will not be zero, but the natural predators will do a better job in the long run if you don't spray.
https://bonnieplants.com/library/how-to ... l-insects/
June-August is the worst time to garden, it is hot and the bugs are in high numbers.
Plant cultivars that are recommended for your climate and plant at recommended times. You can get that information from publications from your local extension office.
Add compost every time you plant to continue to build up the soil. Organic fertilizers like bone meal, manures, take time to work so you want to get those things in the soil a couple of months before you plant. If you use synthetic fertilizers. Only add what is recommended. I would get a soil test from your local extension so you know how much and what you should add.
Now is a good time to start collecting for a compost pile. The simplest pile would be a pile that is 3ft wide, 3 ft long, and 3 ft tall.
You can make a corral out of pallets on three sides. We use a simple plastic sheet that is 36 inches in diameter and 3 ft tall. We have a pvc pipe drilled with holes to provide aeration. We fill the container with chipped mulch that we have stockpiled and alternated with vegetable scraps we get from a restaurant, grass clippings, and fresh leaves. We end with chipped mulch and we water everything well. We cover it with a plastic cover or we could use natural burlap. Once a week we move the plastic bin to an area right next to it and scoop the material back in to the container. We have compost ready in about 6-8 weeks.
In the meantime we stockpile more browns. It works better for us to build it in a day rather than add to it gradually. It starts cooking sooner so it finishes sooner. It is hard to get enough greens if we only depend on what is left from the garden or kitchen and it is easier to pick up bulk waste when arrangements are made with a restauarant. We just have to make sure we are consistant with picking it up on time. We provide them with the buckets. The restaurants don't want to stockpile the stuff either so we pick it up while it is still fresh.
Happy gardening in Hawaii. Gardens are where people grow.