You know it's funny how OpenWRT support never happened for eero. On the patreon when eero was first getting funding one of the selling points was allowing OpenWRT compatibility.
Maybe before Amazon got hold of eero it might have been possible but it must now be locked down super tight.
the SOC/ wireless are supportable (have free drivers)
the vendor isn't too hostile (they can lock down access quite 'well', sometimes there are security bugs that can be exploited, but there it gets difficult)
the device is affordable and attractive (specs) for prospective porters to buy it
someone of those who bought it, will have to do the necessary development work, this requires a floating spectrum between experience and/ or motivation/ dedication
case easy to open, serial console semi-accessible add to the potential attraction
...all of these conditions need to be met, and the porter needs the dedication to successfully finish the porting and have the persistence to get their patches merged.
If in doubt, you will have to be prospective porter, because the device is sitting on your desk and not in anyone else's.
If you want to run OpenWrt, it is usually easier to choose the devices to buy based on the ToH and to choose among those that already are fully supported, rather than to bet on the future or your own development abilities.
There are many decent devices that should be (relatively) easily supportable, but which haven't ended up in the hands of motivated developers yet, be it because of their pricing, regional availability or just bad luck.
I have a 3-pack of eero Pro 6 someone gave to me a while ago. It's been laying in storage ever since because at the time it didn't have OpenWrt support and I needed something with PoE.
After taking another look it turns out that support for another device with similar hardware have been merged rather recently. Maybe it's time to take another look at it.
To be absolutely clear I can't make any promises or set any deadlines at this point.
I've managed to take one apart and I believe I've positively identified the serial console pads. It appears to be using some version of u-boot:
eero u-boot 1.0.1-d99711ac6d-l (Sep 04 2020 - 10:15:37 +0000)
I'm going to solder some wires to the pads since I don't have four hands to hold jumpers and type into a serial terminal at once.
Getting to the PCB is not trivial. You need to remove a sticker, 6 Torx screws of 2 different sizes (!), and the case is snapped together. Fortunately the serial console pads are in a location that does not require removing the large heatsink. You do need to remove the plastic antenna cage to solder some wires onto the console (unless you are really adept with a soldering iron).
I'm hoping there's a closed-chassis way of flashing this device, because if we need the serial console to do that, OpenWrt users of eero Pro 6 are in for some serious disassembly.
Unfortunately the boot delay is set to zero seconds and I'm unable to enter u-boot despite multiple attempts. Or maybe the RxD pin is not where I think it is.
Honestly OpenWrt support is very unlikely here unless I can find a closed-case way of getting to some kind of shell, because opening this thing is much too involved. If I had the time to keep hacking away at this I might try dumping the eMMC contents but right now I don't have a path forward for this.
It does use SystemD on a Yocto reference distro, making this the first time I've seen this particular init system used on a wireless router.
i think extracting the certs from the app and then inspecting the network chatter on a factor-reset setup session would be the easier way for those. Alas, no time for anything.