f you are to have a chance of not upsetting your aunt, you need to tell a few lies here.
Contact a locksmith and have the locks changed, and keep the keys yourself.
Tell you aunt, you have done this, once it has been done and say you have done so to protect both yourself and her good friend from being suspected of removing things from your aunt's house without her permission.
You say she has had cleaners through the years, and quite honestly can you or she be sure that all of these people were honest ?
Even if they were, they may have been naive enough to have keys labelled as to where they belonged and it is nearly always possible to have keys copied.
Having the bank cards changed is more difficult to achieve, as if you aunt is still mentally all there, and you do not say or sound as if she is not, you cannot stop her lending her friend, or anyone else, her bank card.
Would the bank be willing to pay anything related to your aunt's property from an account that only you have access to?
I doubt it is legal to put up cameras in the house or outside it, unless there is a notice stating this - check with the police before considering doing so
Make a list of the things you know or suspect are missing and if possible check with your aunt whether she has disposed of them, or knows where they are.
You say the bank was concerned to realise that an unrelated friend had access to the bank cards and to the house. Perhaps you could dress a change of banking arrangements up as a demand from the bank?
One last point: who is or are your aunt's legal heirs? If you are not the sole heir, you could well be in difficulties if items or money left to others is missing when the day comes for settling your aunt's affairs, which is another very valid reason for making sure that you, who hold the POA , are the only person with access to the house.
Perhaps you need to run all these points past a solicitor of your choosing, not one used by your aunt, so you know exactly how you stand.