Help with choosing steel frame My most recent bike was a Trek Domane SL6 that I loved, but the frame has started to show some cracks and I am looking at switching over the Shimano Ultegra components to a steel frame. 

I am a big guy, 220 lb (100 kg) at my fighting weight so bike weight isn't a big concern. The bike will be used mostly on the road for bi-weekly 100+ mile (160 km) rides and the occasional long distance group ride. (I also might do a triathlon later this year but I am not looking to podium, just finish.) It will also be used to ride for fitness and fun, not really any commuting. 

I have been looking at the Mason Definition 2 frame as well as some of the All-City frames. Does anyone have any experience with these two companies?

The bike will also be used, on weekdays, on a Wahoo Kickr trainer for workout purposes and to build up endurance. 

The Domane is my only experience with modern bikes so I am not sure what else I should look at or avoid, so any help would be greatly appreciated.
Buying a frame to build up when you don't have much experience is tricky because you cannot  test ride frames alone. Most importantly you want to make sure you will get a good riding position fit. If there is a bike store that sells complete bikes with the frames you want try test riding those.

If you cannot test ride frames in this way then you need to look at the geometry. You can basically split geometry into the cockpit dimensions: stack and reach, and the handling: head tube angle, fork offset, chainstay length and front-center. You can use the Domane as a reference point. If you liked the riding position look for a frame with similar stack and reach and remember you can adjust riding position by getting a longer or shorter stem. Longer chanstays and slacker head tube angle will give you a more stable but slower turning bike.

Some other considerations. Most steel frames will have a threaded bottom bracket shell. The Domane has a pressfit bottom bracket (BB90 I think). Shimano cranks with 25mm axles will fit in external threaded bottom bracket cups but I think you will want to get a frame with a 68mm road bottom bracket shell width to fit the road crank axle length. (You can check the specs of you crank here.)