I'm no fit expert, but one thing I notice between my road bike and my CX bike is that I prefer to have less drop between saddle and bars (maybe an inch and a half less), and I also have my saddle about 4-5 mm lower as compared to the BB.

Higher bars and having less weight on the front end makes it easier to go down some steeper descents. A slightly lower saddle allows me to better shift my weight around on descents and corners and generally unweight myself from the saddle when going over bumpier terrain. Also makes it a tad bit easier to flying mount back after the barriers when the bars are a little higher and the saddle is a little lower. Not a big deal on the first couple of laps...but near the end of races and I'm gassed...my flying mount suffers.

Again...no fit expert. I tried to mimic my road position when I first got the bike, but found myself adding spacers under the bars and lowering the saddle. YMMV

I'm 5'8" with 32"(ish) inseam. I ride a 54cm Focus Mares.
I'm pretty sure a 56cm would fit you fine.

The Focus Mares in a 56 is almost a dead match to your stack and reach on the 2009 Cervelo S1 if this link is accurate.

We're (finally) starting to see cyclocross bikes trending a little more towards the mountain bike side of things where they are getting longer in the frame and paired with slightly shorter stems. If you're thinking about more a fatter tired road cruiser/gravel bike than keeping it close to your comfortable road geometry is great. If you're going to be racing cx and/or riding some trails, it could be worth test riding some bikes with a little more reach and head tube/stack. They might not feel exactly like your road bike, but your road bike wasn't made to hop railroad ties or skid down a muddy hilly with your saddle against your sternum. I had the chance to buy a 58 Mares at a crazy price not too long ago and was in dismay at how short the stack was on that thing. It just didn't work for me, but might for you. If that low of a front end is appealing to you, the Moots probably won't be a great fit because the front end is 30 or so mm higher.

Bear in mind that in CX set-up you generally run your brake hoods higher up on the bars (nearer the tops) so that you can really grab the hoods and "drive" the bike so take that into consideration when you are thinking about reach.
I wouldn't go as extreme as 1-1.5" lower saddle to your road bike.
If you're thinking of racing, for frame size be sure you can shoulder the bike easily i.e. get your right arm and shoulder in through the front triangle with ease and at speed - probably not so much of a prob if you are looking at 54/56.

Also there's some good articles out there e.g. https://www.cycle-smart.com/...-cyclocross-bike-fit

I’m similar size. Ride 56 Cervelo P3 and 56 Tarmac. Fit on 54 too. I called Canyon and they recommended small cyclocross bike. Went with that and the bike fits perfectly and I more than love it.

I went ahead and pulled the trigger on the Focus Mares. From what I hear, Focus will not be distributing bikes in the US past this year so finding one was difficult. I'll post pics when I get it.