Stone and concrete or plaster are going to be significant challenges for WiFi in general, but even more challenging for mesh-based solutions, since they usually use 5GHz for the mesh interconnect. Running Ethernet will be painful, though you may be able to get creative by running it outside, through an attic or basement/crawlspace, etc. If you have coaxial cabling in place, you may also be able to use MoCa adapters to create an Ethernet equivalent. Powerline adapters aren't great in my experience, but you may find them to be better than relying on 5GHz WiFi for the interconnect.
All of that said, it's important to follow the connection rules here: https://support.google.com/wifi/answer/7215624?hl=en
In short, the primary must be connected to your internet service via Ethernet. The secondaries connect to it via 5GHz WiFi mesh, but can be connected back to the primary via Ethernet. They need to be connected to the primary's LAN Ethernet port. They can't be connected to the "outer" Ethernet network that the primary's WAN (globe) Ethernet port is connected to.
So, you could use powerline to get from your modem to a more centrally-located primary. But, if you do that, you won't be able to use powerline for anything else, since it would be part of that "outer" network. Or, you can leave the primary where it is and connect a powerline adapter to its LAN port and then connect distant secondary units to that "inner" network through powerline adapters. That won't be fast, but it may be more reliable than trying to make 5GHz work through stone walls.
If you do connect secondaries back to the primary via Ethernet (either directly or via Powerline or MoCa adapters, etc.), it's very important to understand that those wired secondaries cannot act like mesh extenders for more distant wireless-only secondaries. I would try to get all of the secondaries connected if possible, especially in a difficult environment like you describe. When you do that, it doesn't matter where the primary is located.
I hope this helps.
