Earlier quoted context omitted.
What is your house built of? In the UK, we’ve got houses with brick walls everywhere - it’s killer for signal.
I live in a wooden house, which is kind of ideal yes. You say brick, which isn't actually that bad, if you reference the classic NIST report [1]. And even in a brick house a lot of the interior walls are still just plywood or drywall, even in the UK AFAIK. What is interesting though is that OP said it is sufficient with one AP per floor plus one outside. So apparently propagation within one floor is fine, but between…
We’ve got 3 UniFi APs downstairs here, 2 up. There are still things on the periphery that only just cling on, and when you’ve got stuff like that it takes a lot of airtime.