Looks good - simple and solid - I like it!
Any suggestions on how I can improve this set up are welcome.
Here are a few suggestions.
1) Remove the bar clamps and drill the bar so that the bar bolts on from underneath. That way to change chains you would just have flip the carriage over by 90º
2) Seeing as you have a fixed bar length maybe add cranks and a chain that would raise and lower the bar at the same time.
3) Attach some sort of a shield to reduce the buildup of sawdust on the inner track
4) To hold the log, a simple wooden stop attached to one of the wooden cross pieces of the track by a bolt and a wing nut would suffice. The stop could flip out of the way when not needed or unscrewed and moved to a new position for a different length log.
5) When you put it on wheels (or for that matter even now) arrange for the track to have a slope so that the saw and mill goes down slope (less pushing) - or add a crank to pull the saw through the wood.
6) Add an Aux throttle and a extra handle
Here's one of mine
I'd put it as high up as you could (maybe even as high as A) and then add a vertical handle at B.
That way and operator doesn't have to continually have their arms spread apart (I call it gorilla stance) and can then stand up and mill.
Having a choice or vertical and horizontal handles helps give an operators hand a break and any extra weight is not usually much of an issue with carriage type mills.
7) Even at 15" of cut in softwood I'd be using an Aux oil. A CS milling saw (especially small ones) needs to have it's load lightened as much as it can.