Nighthawk AX12 WiFi Router (RAX120) with Nighthawk AX8 8-Stream WiFi 6 Mesh Extender (EAX80)

Doubtful but we can't say that 100% without trying. 

Do you own this flat? If so, a better option is wired ethernet to be pulled. If you don't own it, you could look at MOCA adapters (ethernet over coax) if there's coax ran or powerline devices. They do a pretty decent job but the powerline might not be more than that 300mpbs you're hitting. 300mbps is actually pretty solid through thick walls. Most devices don't use anywhere near that. Streaming 4k only takes 25-40mbps. So hitting faster speeds is really only beneficial if you're moving massive files or just like the speedtest results to be higher. It doesn't actually improve performance a whole lot otherwise.