One product that is made for cleaning the cooling system in place is called Evapo-Rust. They make a product that can be run in the cooling system for a time as coolant. Some leave it in for a month or more and it will leave the cooling system rust free. I am not sure if it is available outside the U.S. It is about $30 U.S. per gallon.
https://evapo-rust.com/The other thing you should consider is a filter in the top radiator hose. There are commercial ones made (Gano Filter), or many use a piece (toe section) of nylon stocking clamped between the hose and the radiator inlet pipe to filter the water coming from the engine. The filter will catch chunks of rust that break loose from inside the block before they get into the radiator and plug things up.
