Does breeding for mutations take forever?

We're currently in the process of taming our ideal Rex. We followed the Dododex Guide and crossbred our high melee high health wild Rex with a low level Rex until we had a min maxed Rex with high health, high melee and low levels in every other stat (we also have one with stamina for riding). Now we created one male with these stats and around 30 female with identical stats and started breeding like hell. We have boosted rates and have an absurd quantity of eggs in no time. But still we are quite shocked to say the least how long it takes to get the right mutation. We already invested hours in looking and so far we have 4 levels into melee. Is there something we are doing wrong?

Once we have a mutation in melee on a male we just exchange the male, all females are exactly the same since the beginning. Is it supposed to take ages? How can we accelerate this process without completely removing a bit of a struggle?