LibreCMC is the freest router firmware out there.
https://gogs.librecmc.org/libreCMC/libreCMC/wiki/Supported_Hardware
The WNDR3800 is the most powerful hardware it supports, but it can be hard to find. The TPE-R1300 can be bought new and has the fsf ryf certification.
https://www.thinkpenguin.com/gnu-linux/free-software-wireless-n-mini-vpn-router-v3-tpe-r1300
