Can the OPI5 be Configured to Run LCNC?

Nice work! I've ever read somewhere that a hardware pwm generates cleaner pulses. It should be possible to use the pwm pins on the OPI too but don't ask me how. But if the RPI already fits your needs then this will do for sure assuming your driver works as expected.
Maybe you could compare the output of the RPI and OPI?

About the protocol of MESA:
forum.linuxcnc.org/10-advanced-configura...ntrol-card-from-lcnc

@kanuboy13b That heatsink does also works nice indeed I bought the the same after reading your message. Bought a pwm fan with it and created a little patch for it using the code from radxa if you're interested I could upload the patch or dtb)

I bought a Bridgeport Boss mill last Saturday, and have been going through all these forums looking for a good SBC candidate to run it.  Based on this thread, and royka's post in particular, I pulled the trigger on an OPi5 last night. Would the Mesa 7i90HD be a good match-up for driving 3-steppers, a 0-10V input to a VFD, and two relays (one for an oiler, and the other for coolant)?

Cool! The 7i90HD should be possible with a SPI connection but it's more prone to failure. I would recommend a Mesa board with Ethernet like the 7i92tm:
mesaus.com/product/7i92tm/

And a bob connected to it for the 0-10v and protection of the Mesa.
aliexpress.com/item/32972731880.html

Or this one which already has isolated in/outputs:
mesaus.com/product/7i96s/

If your VFD has got modbus (rs485) it would be better to use this instead of the 0-10v.

This is the stepper kit I will be driving:
www.omc-stepperonline.com/ts-series-3-ax...y-2m-cable-3-clts120

I haven't ordered a VFD yet. So, I'll look for one that offers modbus as an option.

I was hoping to use the SPI in order to free the ethernet for networking. I could target wireless, but reception not great in the shop, and the discussion here point to more complications with interrupts.

One of the target goals is simplicity of installation. Using GPIO pins direct to the stepper drives would be simplest, except I'd still have to level shift. The 7i90HD would take care of that (I assumed), but now we're talking about opto-couplers, which I thought would be on the drivers.
Maybe I could go with one of the breakout boards from the GPIO pins?
Looks like the 7i90HD is out of stock anyhow.

OK. VFD with modbus.
www.amazon.com/Mollom-220V-2-2KW-3HP-Frequency/dp/B0BM5RRX9M

The last piece will be to get the signals from the OPI5 to the machines that it is controlling.  In order to reserve the ethernet interface for networking, would it be sufficient to add a voltage converter  and then a BoB .?

Ok I get it, although the latency with WiFi was still way below the requirement for Mesa or Remora (200us seems to be usable and kanuboy13b had 7.2us with WiFi) but if the reception is bad an external antenna might be an option or an usb ethernet adapter.

Mesa over SPI should be possible but it seems you'll need to keep the wires short and you would need the dtb overlay in the attachment.

To connect the bob "directly" to the opi5 I believe a level shifter would be enough indeed, Guglielmi might be able to tell you about how he has the stepper drivers connected and if he tested his HAL driver already.
To communicate to the VFD over modbus you also need a rs485 to usb adapter.