For under $30, is the Cudy WR2100 a good device to run as an AP, possibly as a router too in a different setup, or is an old Archer C7 v4 still the better choice?
https://www.amazon.com/Cudy-2100Mbps-1733Mbps-Streaming-WR2100/dp/B089GNFCHS
I'm surprised the Cudy WR2100 doesn't get more mention as a value OpenWRT device.
Well the 30$ price is a temporary discounted price, at that price it's good value. At 80$ it has more competition.
Cudy WR2100 has a better CPU (dual core, similar frequency) I don't know for wifi but I think it is at least on par, both have external antennas and both use wifi chipset with good driver support.
Flash and RAM size are the same.
Only downside of Cudy WR 2100 is that there is no OpenWrt stable release for it yet.
Although you can use the Release Candidate of OpenWrt 21.02 for it (which is one of the final testing versions for the "stable release"), from here
https://downloads.openwrt.org/releases/21.02.0-rc4/targets/ramips/mt7621/
and eventually you will be able to use OpenWrt 21.02 release on it too when it lands.
It's more like $25 shipped today. At this price just grab 2-3 and put APs all around the house if you need coverage. Not sure the price will go up by much from here, it feels like they are trying to clear stock, perhaps in favor of their Cudy AX1800 X6 wifi 6 which is about $50 shipped.
Yes that's usually a good idea. As a general rule of thumb, two APs in different places easily outperform a single AP costing twice as much.
So in most cases spamming cheap APs all over is the best decision