Some used cars come with warranty to cover such things. Do check.

In the event it does not, suggest you visit RBM Radiator Specialist [] , this is a common issue on iRiz and Persona. (Heck, even my car that's fully serviced by Proton also like this, SOP x de, simply add water, compared to older P2 cars at home, till now all good)
Budget standby about Rm300 and go early because theres lots to do. Best you whatsapp or call them for appt as well.

Don't waste your money do the coolant flash. You need to change your radiator.

I suggest you study more first bro, some rads come cheap, some are better made. What you take also if your coolant and water level not optimum / balanced its gonna come back.

Those hose and pathways all need to wash.

I still stick to what i said, if can service get it done, if cannot only change rad.

And rads must be just nice, too cool your engine also wont run optimally

changed radiator after 5 years, which is abt 2 years ago. rusted inside, never add any coolant/water, service in SC only.
cost 620 in klang/shah alam area for radiator/hose/thermostat/coolant change.