Is the oven actually faulty?

Some ovens only use the top element to ‘grill’ or brown dishes on top for example if you want to crisp up the cheese on top your mac n cheese dish and not as a ‘baking’ element. Temps don’t matter as much because the dish would be up close to the element.

You say it has been working for 12 years, does that mean the TOP element has been hitting the right temps for the past 12 years?

Minor dirt, nothing to worry about in terms of temps but don’t let your mom see it XD.

Is there only one thermostat?

If you’re handy/industrious enough you can get a simple thermostat like an EGO 55.18062.050 which are fairly cheap and use that as a diagnostic tool.

If that solves your problem, you can swap out for the expensive part or leave in until it dies.

I’ve had thermostats fail open or closed. Open meaning the element never switches on or closed meaning the element never switches off.

I’m not an expert, just a hobbyist.

Is your thermostat probe in the proper place and the element isn’t hanging off a bracket in the wrong place? Any improper placement of either may be giving an incorrect temp reading on the probe