G/O scale DIY locomotive DCC decoder recommendation ?

It would have to be home brewed just like this HO board is.  This HO boards is the only Arduino DCC compatible thing I've heard of.  But I'll admit I'm not heavy into the DCC world.  I just run DCC at home and have done one install on a locomotive using the ESC LokSound L decoder.  I find the LokSound decoders more then configurable for me I haven't bothered looking into others.  Why would you want Arduino with a decoder?  And reading the page on this HO one, it's not Arduino based, he was just saying it can function with the 'Pro Mini' like a shield does for Arduino.

In my understanding DCC functions like locomotive address change, triggering sounds or lights in the loco follow the DCC signals require some kind of interpreter, which in this case is Arduino processor with its programmed code. This is why I described this decoder as Arduino based.

Strictly arudino based would limit you to DCC plus. Google that and you get the github page. The project has probably been abandoned as it hasn't had any activity in a long time. It does work, I see posts elsewhere of people using it, but you are going to have trouble getting help for getting running. Another option that is under development is ESP32 based, and can be found at https://github.com/atanisoft. Both take some significant amount of work to get going, and are very HW specific. Those are the only two that I'm aware of that are micro based and in shape to use. Hope that helps.

IF you have a full function DCC system, you can program all CVs using the master controller that came with it.    IE NCE, Digitrax, Lenz etc.    You don't need any additional device.    If you want to use JMRI software on your computer to run DecoderPro and do the programming with windows screens, then you need an interface device such as a Sprog, PR3 etc.   Some of the newer released versions of some systems do connect directly to computers too without the extra device.   

I have a SPROG II for programming with DecoderPRo.   I have no idea if it Ardino based or not.    I had not heard that it was.

The basic CVs such as address, lights, and basic speed controls are very easy to program with the master throttle.

As for Decoders, I have not found one  yet that will not allow programming addresses and light functions beyond default, and speed curves and options.     For O Scale, my favorite non-sound decoder is an NCE D408SR.   This decoder has a rating of 4 amps continous and the latest releases have at least 10 amp stall ratings.    They are really hard to burn up!    I have used some small sound decoders for sound only as a second decoder in a loco.   The only high rated decoder  I have worked with for a friend is a Soundtraxx Tsunami 4400 steam.    It is not rated at quite as high current as the 408 but still good.    I installed in an N&W Y3 and the boiler is big enough that I could put the speaker under the stack in the front.

What's the goal here? There are lots of ways to do what you want. I went down the Arduino path, but am moving to a system that uses more expensive, but off the shelf to reduce the effort. I still want to do the ESP32 DCC setup at some point, and communicate with Atanisoft about his work quite a bit. But honestly, given how busy life is, I probably won't ever really create my automated DCC layout using OSS type stuff.

I do have a lot of experience with some of this though, and would be interested in why you are going this route?

My 2 cents.....

I'm not sure what the attraction is for Arduino.  You still have to come up with an additional circuit for motor drive and function outputs.

The basic difference between mobile decoders as a function of gauge is the ampacity of the full wave bridge and the motor driver output.  I have altered NCE HO decoders with different components to run O gauge.

I also used a MicroChip PIC and made my own decoder.  Since space was basically irrelevant for O gauge I built it thru hole instead of surface mount.  Good for well over 10 amps.

And as far as sound goes, I have coded up sound ICs for chuff, prime mover, horn, whistle, and bell.  I don't see the need for a full audio menu.  Personally I've got a 15 minute tolerance of prime mover sounds before they become irritating.

Lou N