Those test strips dont test for ammonia, so you could have a bigger problem than you are aware of. Test strips arent very accurate which could compound the issue.
Before doing your water change remove a sample of water. Either get a more accurate liquid test kit that tests forbpH, ammonia, nitrite and nitrate or take the sample to a fish store and get them to test for you.
After 18 months your tank should have cycled but it hasnt. My opinion is that you have too many fish for a 5g tank. The tank and its filtration simply cant cope with the amount of waste that goes in. You need to either reduce the number of fish or get a bigger tank. In the meantime change 50% of the water every 2 days, and cut back on feeding. Only feed every 2 days until you decide whether to reduce fish or get a bigger tank.
You should be able to keep the 4 guppys ok in 5g, but they would be better in 10g. If you decide to keep the tank, rehome the corys. Corys are simply too big for your tank and need to be kept bigger groups than 2. If you decide to get a bigger tank, then 10g minimum, but it really depends on the type of cory. Unless its a pygmy cory or some other type of miniature cory then a 20g is the smallest size i would recommend and then getting another 4 corys will make them happier.