There is actually no way to avoid this, aside from setting the Fee nearly to the reward.

It's essentially MitM all the way down.

even the private mempool can attempt a double-spend with a larger fee, get one transaction ahead, then try to maintain an edge long enough to be the "longest branch" for consensus - the 51% attack only needs 33% in reality, much less when your the private mempool that can take advantage of the birthday paradox to jump two blocks ahead.

you have to literally mine your own coin with the reward transaction included.

of course, zpk+ would solve this issue entirely.

Alice and Bob wouldn't ever doubt each other again.