Low throughput with Wi-Fi extender

I want to wirelessly connect an OpenWrt router to my main router and use its ethernet ports and potentially wireless interface to extend my network. I am using a Linksys EA4500 v2 that I flashed with 19.07.5 (openwrt-19.07.5-kirkwood-linksys_viper-squashfs-factory.bin)
I installed luci-proto-relay and followed the instructions here for setting up a bridge.
It's working, but I'm getting about a 28 Mbps throughput up and down, even though the wireless page of the router claims it's connected to the main router at move than 300 Mbps.

I've tried using both the 2 GHz and the 5 GHz radios on the router as clients and I get similar throughput on both bands. I'm disappointed since my internet service is 200 Mbps and i was hopping to at least get that throughput. My phone sitting at the same location as the client router is able to connect to the main router and get over 200 mbps, so I don't think it's a signal strength issue.

Any suggestions on how to improve throughput or explanation of why it's so low?

Here is a screenshot of how my interfaces are configured and wireless client connection info