The archer c7 is a great choice for now, but if you upgrade to 300 it won't do SQM at speed anymore. See Comparative Throughput Testing Including NAT, SQM, WireGuard, and OpenVPN for benchmarks on various router chips. If you have dead zones do straight for another router to use as an AP. Maybe get a C7 today and when you upgrade to 300 start to use it as a AP and get a more powerful router at that time?
It sounds like your main router will need to be an r7800 or wrt3200acs if you were to buy today. Warning, the wrt3200acs has buggy closed source WiFi drivers, so it's not a great choice unless you've got another AP and it is very cheap.