I did some research on possible replacements for the MyNet N600 nodes and here's what I came up with so far: For a price comparable node the best I've found so far is the TP-Link TL-WA801ND: https://www.tp-link.com/us/home-networking/access-point/tl-wa801nd/#overview
It is $25 new shipped and can be found for $20 refurbed. The downside is that it's 2.4 GHz only rather than the dual-radio 2.4 and 5 GHz in the My Net. The upside is that it has external antennas with a standard plug so it's easier to attach directional and home-made antennas + it should get better range as is. For folks that don't want to go with the absolute cheapest possible the TP-Link Archer A6 looks good: https://www.tp-link.com/us/home-networking/wifi-router/archer-a6/
It's $50 new shipped. External but non-detachable antennas. 2.4 + 5 GHz dual radio. All gigabit ethernet ports. It looks like it's a re-branded (new but largely unchanged) version of the Archer C6 v2: https://openwrt.org/toh/tp-link/tp-link_archer_c6_v2
These aren't new routers though. I think they're from 2017? So may also be discontinued fairly soon. If we go the route of being hardware agnostic as we've discussed then I don't think that should be a big problem. I ordered one of each.
There's also going the route of a single board computer like the NanoPi R1 or R1S or R2, eg: https://www.friendlyarm.com/index.php?route=product/product&path=69&product_id=248
Cheapest one is $20 + shipping. That's without a power supply. These are 2.4 GHz single radio, single antenna and one of the two ethernet ports is internally connected via USB 2. These aren't officially supported by OpenWrt and probably aren't going to be very performant or stable at networking but I could be wrong.
There's also a similar looking but OpenWRT compatible and more network oriented $20.50 device called the GL.iNET GL-MT300N-V2: https://www.amazon.com/GL-iNET-GL-MT300N-V2-Repeater-300Mbps-Performance/dp/B073TSK26W/ref=sr_1_1 again power supply not included
I might have found a better option. The Xiaomi Mi Router 4A: https://www.mi.com/global/mi-router-4a/
also known as the Mi Router 3G (actually that version also has USB3 and seems discontinued).
$24 shipped for 100 MB ethernet version and $30 shipped for gigabit version: www dot aliexpress dot com /item/33059029344.html
Here's the info for the gigabit edition: https://forum.openwrt.org/t/xiaomi-mi-router-4a-gigabit-edition-r4ag-r4a-gigabit-fully-supported-and-flashable-with-openwrtinvasion/36685
It seems like there are [still some issues with this chipset https://github.com/openwrt/mt76/issues/391] but they will likely be sorted out. The good news is that this is a fairly new router (early 2019 it looks like) so maybe it will stay in production for a while.
I ordered one of the gigabit units but it will probably take 4-5 weeks to ship. Be aware the the 100 Mbit version uses a slightly different chipset but is also supported by OpenWrt (and in fact probably has better support).
I also looked at the Ubiquiti Aircube ISP which is cute and supported but $30 and 2.4 GHz only with no external antennas: https://openwrt.org/toh/ubiquiti/ubiquiti_aircube_isp
If we need a stable high quality high-speed device for e.g. a gigabit link then we might want to go with a Ubiquiti ER-X or ER-X-SFP: https://openwrt.org/toh/ubiquiti/ubiquiti_edgerouter_x_er-x_ka
These don't have any WiFi but have more ram and flash than other devices in this class + the SFP option is great if we need fiber optics.
The ASUS RT-ACRH13 is $70 shipped and has a really good reputation for high performance, probably because of its Qualcomm ipq40xx chipset.
You can buy refurb/used Linksys EA6350 for $35 which also have the ipq40xx chipset but that model seems to be recently discontinued otherwise
Also the MikroTik RBcAPGi-5acD2nD (aka: cAP AC) uses that chipset and costs like $75 but has the round roof-mountable form factor while still having 2 gigabit ethernet ports. This could actually be a great solution if we just want a single rooftop link and a great single AP to cover a room or two.