The “lolly-pop” sprue on production rings is pretty traditional, because you can get away with it, not because it’s the best from a casting standpoint. The only real advantage to it is it’s quick. Sometimes you get porosity, sometimes you don’t.
My trick when I was doing a lot of ring molds in vulcanized rubber was to get some 1/8" brass rod, and depending on the nature of the master, either solder it on, or epoxy it on, in whatever manner I wanted the final ring to be sprued. Then lead that back to the injection button. Sometimes I’d use 3/16", or whatever seemed right in terms of the thickness and cross section of the piece. A more ‘deluxe’ version would be to use silver rod. Brass reacts with vulcanized rubber, and gives a rough brown surface to the mold. Silver doesn’t, so silver molded sprues are smoother, which does actually make a difference. The other advantage to molding your sprues in is that way you can focus on getting the joints at the model, and in the spure (if there are any) clean, so they cast smooth and fast. Do it once, and you’re done forever. Just shoot and tree.
It’s also handy in that the sprues you need for metal are going to be much more than you really need for wax, so the wax will shoot easier too.