I appreciate your consideration for not wanting to flood the market with new coins, but if this is a project that you really want to pursue, go for it!
I personally like the idea of a canine cachers series (being Cache-Hounds, of couse we'll like anything dog-related! ) and would like to see more of these coins - quarterly would be great! One suggestion is to send an e-mail to those who ordered the first edition so that they don't miss out on part of the series.
But, ultimately, it should be whatever you feel comfortable doing.
By the way, the pictures look great - can't wait to see these in person...