I was able to get some Linksys E8450 routers for cheap (~$40 each) on eBay a couple years ago, but they are more expensive atm (but $70 each, which is cheaper than a lot of other options). They require a bit of work to get OpenWRT installed, but it's not bad once the work is done (I have received nearly 400 Mb/s when I am close to the wireless connection sometimes, which is what my internet is capped at). I have used them as a mesh with one as a main gateway and the rest as access points with additional Ethernet ports. They do have the OKD issue fixed now, too, and they're also Wi-Fi 6. I'm looking into getting a Banana Pi BPI-R3 or R4, though I am not sure how well they are with mesh specifically, but have seen a few forum threads of people having some issues with mesh on these specific routers. I hope to switch to fast roaming (wired backhaul) eventually.