Welcome to the forum, feel free to ask questions here.
Personally, I would start with the cylinders. Delete all but the top faces. Then cut the center one into quarters and just build 1/4 of the caster base by combining the quarter face and the whole face and then use a combination of bridge and append to polygon and extrude and target weld verts to fill in the gap and go from there... From there it is just a couple extrudes to get to here ...
the slit poly tool should have worked that is how I cut it into quarters. But the cut faces tool will work it's jut you may not cut right through the verts.
After bridging the three edges. Then pick the two edges of the quarter circle and extrude them out. Then I straightened the edges and inserted som edge loops then connected them up. I'll try to put something together for you to help explain it better
Hey, i watched your video. What is strange to me is that its all quite high polygon. I guess thats fine if you are only modeling this.
But then the wheel has less polys than the bolt thats holding it in.
Also you talk about keeping things quads, but you end up with a 20 sided polygon in the middle circle bit. I don't think its a problem tho.