Your tank is on the minimum size for ten angelfish to grow properly and have a good water quality, so i do not agree to have the discus with them. If you can spare another tank for discus is preferable, the corydoras are not a problem because they are small, one of the main reasons for a ph crash is too much fish stock for the tank water, if i was you i will opt for eight angelfish in your tank and leave them to grow in a good size, i prefare to have better sized fish then more fish but smaller,a good size adult angelfish is magneficent in an aquarium, not to mention that when breeding occures they become territorial and space is needed. Another important thing to prevent from ph crashes is weekly water changes, you can do 25% weekly.
Make sure that you do not have strong current in your tank from the three filters that you mantioned because angelfish and discus do not like currents in the water they feel much better in very slow moving water, in their natural habitat they are found in flooded areas and still waters.
Can i ask you if you use tap water or water from a well for them please?
GOOD LUCK.
Ranchu lover.