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These articles should help: https://www.gardeningknowhow.com/edible/vegetables/beans/harvesting-beans-when-do-you-pick-beans.htm
https://www.gardeningknowhow.com/edible/vegetables/corn/harvesting-bean-seeds.htm

Round to angular streaks on leaves, reddish brown to black; sunken water-soaked areas on pods; pods shrivels and become watery?
Anthracnose is a fungal disease. Remove and discard infected plants. Avoid working in the garden when it is wet which can result in spread of spores. Keep tools clean.

Bitterness in vegetables can be caused by several things. Most obvious would be harvesting too late, but you seem to be picking the beans in their prime. Heat, drought, nutrient poor soil are 3 other causes for bitter vegetables. You can't do anything about the heat, but supplemental irrigation and appropriate fertilization are easy fixes. If you need to do a soil test, contact your local agricultural extension office.

The following articles should help: https://www.gardeningknowhow.com/edible/vegetables/vgen/choosing-the-size-of-your-vegetable-garden.htm, https://www.gardeningknowhow.com/edible/vegetables/vgen/family-vegetable-garden-size.htm

White mold is caused by the soil borne fungus Sclerotinia sclerotiorum. Also called stem rot, many vegetable crops are susceptible to this fungus. It requires an abundance of moisture to grow. Therefore, keeping the area as dry as possible and growing plants in well-draining soil is important for prevention of this disease. If the area is too wet, it could be the result of too much irrigation or poor drainage.

It sounds like you have fungus gnats. This article will help: https://www.gardeningknowhow.com/special/children/fungus-gnats.htm Neem oil will help take care of these pests but will not hurt the plants. Here is more information: https://www.gardeningknowhow.com/problems/pests/neem-oil-uses.htm The green beans may not be getting enough nutrients. Make sure the soil is moist and try giving them a bit of slow-release balanced fertilizer at half strength. Also, beans do best in soil that has been inoculated. This article will explain more on that: https://www.gardeningknowhow.com/edible/vegetables/beans/soil-inoculants.htm