Cheap and small device for a VPN gateway?

I'm making some site-to-site (wireguard) vpn tunnels to connect some family/friend houses to my network. I'm looking for the cheapest (<25$ per piece) and smallest devices that can (or will be able to) run OpenWrt. The only requirements for each device are: at least one ethernet port, and the ability to handle 10Mbps of wireguard traffic (e.g. a full hd stream over wireguard).

So far, my research has resulted into the following contestants:

My questions are:

  1. Are there any other OpenWrt capable devices in a similar form factor and price range that I can look into?
  2. So far from the above list I'm leaning towards either Orange Pi Zero3 or Radxa ZERO 3E due to the price / performance ratio. Does somebody have any opinion on the performance of H618 vs RK3566 for network / encryption (wireguard) operations?
  3. If I go with the Radxa ZERO 3E device that doesn't have a premade OpenWrt image but its processor (RK3566) seems to be supported in other similar boards, how hard will it be to add support for it (even contribute back to OpenWrt mainline)?

You want a kit to assemble or complete contained device?

I don't know about Mango (v2), but I own the MT300N v1 which is the previous version of Mango, very similar CPU architecture, and I also have the Buffalo WCR-1166DS (same CPU as Mango), you can check out this Wireguard test DB, the speed is slow, and since the CPU load was very high occasionally it just hangs, so I don't really recommend it to be VPN gateway.

I don't mind the device being a barebone/sbc or needing some basic soldering, but lets not go to the other extreme of taking bare SoCs and having to design / print pcbs for them too sit on

Your requirements are not very clear. Anything refurbished 16/128 (oe 16/64) will fit your slow VPN requirement and have gigabit port and AC wifi for all other uses.
If budget is not that restricted aim for something useful in longer term
https://openwrt.org/toh/views/toh_available_16128_ax-wifi

There's also the Gl-Inet 750-series.

My main constraint is the form factor; these devices are gonna be hidden behind some consumer (non-OpenWrt) routers. I only need them to have one or two ethernet interfaces, and a performant enough SoC, wifi is unnecessary. There are lots of cheap devices (as you said anything 16/128 refurbished) that can fit my usage, but almost all of them are bulky full wifi routers.

zyxel ubiquiti mikrotik for wired supported routers.

Yes I've seen these as possible devices too, but still they are ~50$ per piece and I'm trying to find something on the ~25$ range