Echoing everyone here, I have a mesh setup in my house with thick plaster and lathe walls, and it’s good enough.

Problem areas:
Outside. We have aluminum siding, and it helps and hinders. Signals outside the house can’t really get in at high strength, so the Wi-Fi frequency range is clear. Same problem though, I don’t get signal in my backyard. This was fixed by putting an access point near a window.

My first floor ceiling is metal. (Looks like copper underneath the paint). Signal doesn’t want to go upstairs. Solved by putting access points near the stairwell both up and down.