Some bubbles is normal? or so I thought....

Try putting some block test fluid through it and see if it changes colour, should tell you if you have a HG/problem.

Maybe worth draining coolant and refilling making sure theres no air in the system..... worth replacing thermostat for the cost at the same time.

Check the pipes around cooling system, maybe drawing in air somewhere..... are you only getting bubbles in the overflow tank and not the header tank?