Look at https://forum.openwrt.org/t/nanopi-r2s-is-a-great-openwrt-device/65374
the nano[pi r2s might be up your ally then, it is cheap and should allow some things up to 700mbps. BUT it is not a real OpenWrt supported device yet, and I do not believe it does wifi at all, but you could try to re-purpose your old wifi-router as access point?
But in general I agree with @slh, it is hard to do Gbps-class networking at your desired price point.