Firstly, apologies, it may sound like critisism (it does)but many of us have been there , been through the same and have got the t shirt.
But there are things that have to be sorted or understood so as not to keep having the same issues.
500g is big, and good, it’s better than 400g and much better than 300g……… but it’s not as good as 1000g and no where near as good as 2000g. We each have what we can afford or fit or maintain, etc.
when it comes to monster fish, overstocking a small tank or pond IS NOT a common practice to keep down aggression and is definitely not “ needed” and anyone who tells you it is, no doubt has too many large fish in too cramped conditions and is trying to justify doing so to themselves. That approach should be kept for malawis, not monster fish. That will only bring more problems. Any really territorial monster fish will see 500g as all theirs so any amount of other fish won’t help. It may help stopping one getting all the bullying but they will just all live in fear instead and the only one happy is the owner ( until the problems start).
Then when it comes to filtration you need to have the right size filter for a) the stock you have and b) the pump you have.
It is common to have too much filtration, when keeping monster fish, which is not the same as too much water flow, nor is it the same as too big a turn over of water. It is possible to have too much water going through a filter and in turn to stop it being able to perform correctly. Increasing the amount of flow through a filter is not over filtration. This would be having a filter that is rated much bigger than the amount of water you plan to put through it or having a filter and pump for a much bigger pond. The problem then is just down to fish being blown around if the water movement is too much.
Having too many big fish just means that when things do go bad, they go bad fast and are harder to fix. If I remember correctly you had the fish in a 500g glass tank the last we saw, is this the pond you are now mentioning?
If not, then is it possible to split them and have half in each?
I know you mentioned breeding red tails at some point, which will mean getting the two you have (if a pair) to 3-4 ft each, and obviously that’s gonna require something upwards of 3000g to do so getting filtration and stocking right now will help them grow up healthy for your future plans.