why can't i get more than 150 mpbs down in seattle?

My home network is incredibly slow and I'd like to know why.

I live in West Seattle and I'm on 1200 mbps down plan. No voice, no television, no mobile. Just internet.

Wired equipment, including the gateway, gets 150 mbps down max, and wireless stuff gets 8 to 10.

I can monitor every device on the network and none of them are doing anything that's eating up bandwidth.

Equipment:

There's a coax amplifier in place. I don't like it, but xfinity installed it during previous ownership, and service doesn't work without it.

I have an older cable modem (an Arris SB6183, which is on the supported modems list) which is only DOCSIS 3.0, so that caps it at 400. I'd like to upgrade it if I can actually use the higher capacity.

I have a fairly nice setup with ubiquiti equipment in my home - two wifi access points, two in-wall ethernet and wifi access points, two outdoor antennas, and a bunch of managed switches.

Config:

I've setup 4 different wifi networks so that guests and renters have one, the kid has one that can be throttled for his gaming, IoT devices get a heavily throttled network, and my wife and I have our own network.

Maybe it's a limit on my neighborhood? If so, I'd rather stop paying for a higher speed plan.

Any ideas or help welcome.