You need to use high end wifi adapters to be able to get close to 500Mbps on Wifi.

USB adapters will not cut it. You would need a PCIe based Adapter.

Wifi AC will get around 50% of the speed of rated link on a good day, worst around 30%. With the given TM C1200 i get 40% of the 867Mb/s link speed on wifi. Check what rate your wifi syncs at, expect to get 40-50% of the rated speed. I once stress tested the AC68U with a dual channel wifi AC with a link speed of 867Mb/s i was able to utilise 90% of its bandwidth but i get a lot of packet drops in the process, and this was really stressful, basically using all of an i7 to push/receive lots of data through.

You can get 3 or 4 channel wifi AC but your speeds will be 50% of the link speed for the client. Phones typically use one channel (433Mb/s), laptops usually 2 channels, so if you get a 3 channel adapter thats 1300Mb/s, 4 channel would match your speed  as the 3 channel practically will give 650Mb/s.
Nothing to do much to do with the wifi adapter, just practical speeds from link speed.

4 channels are typically either 4 channels or 2 channels twice especially with MU-MIMO. at 866Mb/s your expected practical speed for the client syncing at that speed will be 433Mb/s on a good day,

no, PCIe adapters dont actually get better speeds than USB adapters. As long as the interface and CPU arent the bottleneck, PCIe adapters require a sizable external antenna and a lot of internal shielding as well but its hard to fully shield the entire card, so you will not get those higher rates. The sizable external antenna gives the PCIe card better range, but not better bandwidth. Stick with your internal/USB adapters, you can even get usb adapters with external antenna too. This was actually tested and this is why the PCIe adapter is not very popular and price per benefit too.
USB OTG and attach wifi adapter

I would say it depends on the interface between radio and the innards of the router. Some routers connect wifi to the switch, some go through the CPU so share the 1Gb/s during high traffic (if WAN-LAN use 1Gb/s, then wifi have no bandwidth) But most of the time speed is just limited by your sync speed. What  link speed you get on wifi for AC, expect 50% of that on a good day. For wifi N, expect 30% of link speed on a good day.