DO NOT buy the WRT 3200 ACM if you plan on installing OpenWRT, it has issues and either WiFi or something else will not work properly, we spend hours if not days to figure it out with professional IT and came to the same conclusions as some online posts and tickets, it's not working.

WRT 1900 AC works fine, it's was I still use, but it's a bit outdated. Omnia Turris are top notch but using a special OS with OpenWRT in the backend and an additional UI on top, not sure that it's 100% open source, to be checked. You might be able to install only OpenWRT but last time I check it was not officially supported. Beside that it's the dream hardware, 5 ethernet + choice betwen RJ45 or SPIF for WAN, so direct support of some FTTH providers, power full CPU, upgradable hardware, ... and it's made by a EU (CZ) company, not US or Chinese.

Lastly, there is still the option to turn a MiniPC (even an old cheap on) into a custom router, there are some video online from Wolfgang and others, it's worth checking this option out.