Take a look at <https://www.gl-inet.com/>. These boxes come with a nice, simplified GUI, but will run stock OpenWRT, which you can build yourself if you wish.

I placed one of these boxes behind my ADSL router to do load-balancing. To use it as an Internet router, you'd instead need to couple it with a modem. A colleague whom I trust recommends Draytek kit but (a) I've not tried it, (b) it's not cheap and (c) I'm just a stranger on the Internet, so read around carefully before splashing out.

i didn't realize it, but it seems they provide the bootloader sources which most "open source routers" don't. If they also provide the full board schematics then I would consider them completely open source.

imo it is rude to ask for schematics, the experience in open source hardware is that the only people who use it are the vendors who copy it verbatim and then compete on price, while adding nothing. Even the bootloader is a big ask.

Not fully open source in the sense of only libre ISA and Firmware, but there are some open router, firewall and similar devices developed by CZ.NIC (Czech domain name registry):

https://www.turris.com/en/

Depends on

a) what your WAN connection is and b) how open you need it to be (i.e. there are no open hardware wifi cards)

My WAN port is Ethernet (the GPON conversion is done in a box I don't control). This has the advantage that pretty much any OpenWRT router will work.

I currently use a Traverse Ten64 (disclosure: received the unit for free as part of a beta test) which works reasonably well.

You could also look at gl-inet as someone else has mentioned, or the Turris range of routers. I would not be bothering with flashing a commercial router with OpenWRT anymore.