Installing OpenWRT on a device obviously gives you greater freedom and control over your networking. However, there is probably still a difference between the supported models in terms of the highest level of achievable freedom, how much proprietary code remains after installing OpenWRT and how likely it is that the manufacturer installed an unremovable backdoor.
I found very little information about this. So far, it's the following:
OpenWrt replaces kernel and rootfs, the OEM bootloader usually remains (exceptions exist, e.g. the belkin rt3200 or some lantiq devices), as do some 'data' partitions (such as wifi calibration data).
For years Broadcom didn't supply enough quality FLOSS drivers (or hardware specifications) and it seems won't ever happen, proprietary wl drivers often causes crashes or incorrect behaviors, then avoid Broadcom's wifis. As recommended by many people, Atheros or Ralink/Mediatek wifis are probably the best choice.
there are FOSS drivers for Texas Instruments/Infineon/Lantiq Modems in these SoCs: soc.lantiq!
This forum post is similar:
Well, ath9k is pretty much the only exception with no actual WiFi firmware blob. Pretty much all current alternatives require a firmware blob.
Mediatek mt76 has probably the smallest closed-source firmware blob of the current main alternatives, and is seen as the good guy on the block.
Furthermore, I found some references that the Banana Pi router boards and the Mochabin-5G are freedom-respecting, but without much further elaboration, and I consider these models niche.
The there have been forum discussions like this one, but they don't contain too much practical information.
So, with that said: which manufacturers and models do best at respecting the owner's freedom and privacy?