Galaxy tire pressure warning light issue

Sounds like it. If the pressures are all good, but you're still getting warnings and error codes, it almost certainly points to a faulty tire pressure sensor. How old are the sensors? Have you ever replaced them?

I've never replaced them since owning the car, and I've had it for 5 years! So I guess they are pretty old. But why would an old sensor cause a 'too low' warning? Wouldn't it just stop working entirely?

The sensors have batteries inside them that power the signal they send to the car. Over time, those batteries die. When the battery gets weak, the sensor can send incorrect readings before it completely fails. That "too low" warning is likely a symptom of a dying sensor. Get them checked at a workshop, it's probably the sensor battery. It might be time for new sensors anyway.

Cheers for the advice! Took it to the workshop and you were spot on. Turns out the sensor batteries were indeed empty. Cost me 280€ to get them all replaced. Annoying, but at least the error messages are gone!