For several years I've used a straight-grain 5/8" dowel from the local hardware store. Only modifications were cutting to 40" length to give me a good 12" beyond my 28" barrels - just cuz', gluing a 1/8" thick piece of leather to one end (thought it might be a good idea - don't know if it helps with anything or not), and sanding the opposite end to a nice rounded profile. Works well. I believe the wood is poplar.
Regarding the concern for breakage, it would take quite a bit of force to break a 5/8" dowel I believe. Mine doesn't really bend much at all. My thought is, if you are pushing on the dowel so hard you are concerned about breakage, something is very wrong. Perhaps wads that are too large (as in - WAY TOO large) in diameter for your bore.
The dowel is not practical for use as a cleaning rod in my experience, unless the business end was reduced in size to resemble a cleaning jag. I didn't do that since I always have a segmented rod available for that chore.
At any rate, enjoy your experimentation.