We used 2.5" 3D printed idler wheels on our track system this year. They held up better than expected and were very light weight. Our test wheels didn't have bearings and the later ones used 4 skate bearings to help support the hub. Without the stacked bearings the wheels would waller out quickly. We also used some large 6" 3D printed idlers on the prototype. They used two flange bearings and held up fine.
We replaced the large printed idlers with turned aluminum. I'd recommend turning your blanks and then machining the features. I blanked the idlers at work and then had the students mill the bearing pockets. The lathe at the school is pretty tired. I uploaded the video of the process. The students had to mill the soft jaws before milling the wheel.
https://www.youtube.com/watch?v=8Y0hI6mcwCg
We also modified some COTS idlers, these videos may give your some idea of the secondary operations.
https://www.youtube.com/watch?v=WkNYmihLryo
https://www.youtube.com/watch?v=EDhaW5d9uDk
If we need to ever make a lot of wheels we'd likely mill out a pattern and cast the blanks several at a time from scrap. Material cost on large wheels can add up quickly as round bar is typically a few bucks a pound. I think the large idlers were about 7 lbs each in material. We made four.
A method that I'd like to try would be to mold some from castable plastic resin in a silicone mold. Any teams casting them in plastic?