homelab

No not really. Often the processors are the same (though there may be versions of micro desktops that have power focused processors like deletion or atom). These are mostly the same setup and the biggest difference is the form factor itself and (because there is more space) serviceability for adding (ie: pcie slots etc)

The sff often have 3-4 pcie slots and integrated psus that are a little more impervious to power fluctuations because the psus have bigger caps. The micros generally have the pico psu style (aka laptop style power brick) and no pci slots (the m715q/m720q think center basically being a unicorn in that is does have a pcie riser)

A dedicated router will mostly be idle (in comparison to a desktop workload) and since they are more common than the think center. Probably easier to source and cheaper.

The m.2 A+E card/adapter replacing the wifi card is new to me. Very cool.

Wow, that's cool. Is that an Intel based nic, driver support is good?

With what @infinitevalence recommended I think the M700 is a good inexpensive option if you don't mind doing some work on the case to hack in another ethernet port. Something like this would even get you 2.5Gbe: https://www.aliexpress.us/item/3256804495748525.html

It's Realtek, but there appear to be OpenBSD drivers. That exact one will not work with the headers hanging off the back of the card (that end of the card is right behind the power button in the front corner of the PC). But if they were desoldered and replaced with vertical headers it would work perfectly. There might be something similar with different connectors, or a 1Gbe Intel card available.

I have a spare M700 that I just opened up to take some measurements. There is ~4mm space above most of the m.2 wifi card with the drive caddy installed, or ~18mm with the caddy removed. Even with the caddy installed there is that ~18mm open space above the back 5-6mm of the card, so a vertical connector there would work , but would conflict with the install/removal of the drive caddy since it slides toward the front of the case to remove. further modification would be needed to use them together.

At the back there is ~50mm side-to-side space between the fan exhaust and ethernet port. Only 32mm if you only take up the space of the accessory video port. In that width you have ~15mm vertical space (above the row of ports at the bottom), and ~32mm depth (from the inside of the case back to the back of the SATA connector for the 2.5" drive). You would basically just need to enlarge the optional extra video/serial port opening.

You could even fit a multi-port interface at the back, up to 50mm wide. But you wouldn't be able to use the case screw any more since it's in that space. And you would need to cut the tab it screws into off the case's top cover. But the case should still close ok because of the way it slides into place.

Now I'm thinking about ordering that adapter for my own machine to hack in a 2.5Gbe port.

The m700 is a fine box, but doesn't have the PCIe slot for an add in nic. This would limit it's utility as a router box. Even a m720q with a pentium would work well as a router box.

Youre right, The 2 port vaults are $176 which is almost double your budget. I figured you could find a used one for cheaper, but I just looked around and you cannot :(

You can check out ODROID. The ARM version for $83 and a x86 version for $129. You can play around with specs and models and get those numbers down lower. Pretty large community playing with them, BSD probably can support them or at least the info is most likely in a forum somewhere.