For building penetration you need low frequencies but those low frequencies have less bandwidth available because the bands are smaller. At the same time bandwidth needs rapidly go up, making those low bands fuller. Because there's a limit to how many bits per second you can get out of a Hz of spectrum.
What would really help is the easy provisioning of signal boosters. A bit like the sure signal thingies that Vodafone used to sell but multi provider ideally. This could be incorporated into the standard. Then establishments could easily include a booster just like many of them offer free WiFi now.
They're still readily available.
But yeah think this would need a legislative push to solve. Force high rises to host multi provider relays as you say and hook it into their fiber links.
But that's also a physics thing. Plain same-frequency boosters hamper frequency reuse on neighbor cells, so less bandwidth for you. And if boosters run on a separate band, you'd need to do frequency planning for them just like it's done for cells now.
Wireless is finicky, we should really avoid using it everywhere wired can be used instead.
/s
Ex 6G = 8B.9 verizon cool blue and 7G = A0.D costco ultra red or similar.
(/s)