I need a Wireless Engineer

We have had an old house that has thick walls that makes it hard for Wifi to go through. In addition, we have a second building (containing an apartment and a study) on the property that requires Wifi. The building is about approximately 120ft away from the house. Our current router is Xfinity and is in the building. We also have an AmpliFi Dual-Band Mesh Wi-Fi System, and a Linksys RE9000 Extender. The Wifi is overall not terrible, but it's not ideal. There are some weak spots, and doing things that require a strong connection such as video calling, are somewhat difficult if you are not close to the main router, or one of the Amplifi's. I don't know what the best solution is to make the Wifi really good everywhere in the house, as well as the other building, so I'm hoping an expert could help us find the best possible Wifi solution, by either coming and accessing the Wifi in person, or virtually. I just feel like there must be a better way to have really good WiFi, and I am not technically savvy enough to figure that out by myself.