With Broadcom it always seems such a struggle (no mesh support, no RSN-IBSS, MAX MTU only 1500, driver issues), for all their wired and wireless gear I've encountered, that I gtfo quickly when I have the choice to not use that HW. Same with NVidia. Result: Hardly ever driver problems on Linux and Openwrt.

And everybody know it, but somehow these devices always are at the top of some consumer recommendation list, and people keep plugging rpi4s etc everywhere.

Wifi will always be hit or miss with Openwrt. The general advice is to use openwrt for routing and a separate access point that runs vendor firmware for wifi. Should you want everything in one device, R7800 is a well supported openwrt all-in-one device.

I have two Ubiquiti AP-AC Pro APs, and I've been running OpenWRT just fine on them for the past year and a half. Completely rock solid. I didn't do performance testing with the stock firmware, so I don't know if I get the same WiFi speeds, but what I do get is more than sufficient for my purposes.

Maybe other hardware doesn't work as well, but I think if you choose carefully, it's fine.

Wifi is only hit or miss because it's often difficult or impossible to tell when a vendor has silently swapped out the guts of a model and replaced them with Broadcom parts.

How does dd-wrt pull it off, then?

(As far as I know:) By sticking to the Vendor-supplied SDK kernel version, with ancient blobbed up 3.X Linux kernels, for architectures, where modern kernel/module/userspace (source) is not available.

Openwrt aims for "near-mainline" blob-free kernel, currently on 5.4, with 5.10 in current master for many architectures.

The TP-Link Archer A7 is a decent entry-level device: https://openwrt.org/toh/tp-link/archer_a7_v5

I've had good luck with the Netgear R6220.