It's a queue, but not a first-come first serve one. The problem I imagine is that, once you set your transaction fee, and it turns out to be too low, it is hard

It's a queue, but not a first-come first serve one. The problem I imagine is that, once you set your transaction fee, and it turns out to be too low, it is hard to retroactively pay more fee. Just because the system doesn't allow two spends of the same output in the mempool at the same time, and canceling a transaction is hard.