Currently I have 4 tanks. 2X75 gallon 2 oscars raised from more than 1year and 1X30g palnted with 6 neon tetras, 1X10g planted with 4 tigers barbs. I m into this hobby for about 2years. I really love the oscars. I have learned to keep water quality stable.
*****Sounds good. You should have some basics down. If you don't do it already, I'd recommend buying a water test kit. The liquid kind with test tubes. Not dip sticks. API freshwater master test kit is roughly $20 here in the US.
I am using external filtration which runs about 500g per hour.
*****Need more info on the filter size and media. 500 GPH will not do. You need at least 5 times an hour turnover rate, better 10. Which means at least 2,000 GPH pump, better 4,000. At these rates, you can get away without an additional aeration.
As i learned about monster fishes, I have to do 30-40% water changes weekly. Correct me if i am wrong.
*****It really depends on the bioload and how high nitrates would get. Initially even less could suffice. When fish will have grown, be prepared to possibly change more water, like 100%.
It is an outdoor pond. pH is 7, present temp is about 23-26C.
*****How are you going to handle yearly temperature seasons? Also, how will you handle algae? Overheating, if in the sun?
Will add lighting and driftwood for hiding. And some flowing plants like water lily.
*****For an outside pond, I'd not think you need lighting, just very dim night light would be beneficial. Catfish in general dislike strong light. Driftwood is good but also impairs your ability to observe the fish and uneaten food, if any, also takes away from swimming room, which will be more of a problem later on. Water lilies would provide the shaded areas but with the flow rates I recommended, they may not be possible to have.
*****I note you said nothing on the hardness. I think it'd be a good idea for you to learn to use the hardness liquid tests, both KH and GH. You will learn about your water and also it will enable you to be on guard against pH swings and pH crash.