This is a very sensitive subject and I don't think theres a right answer.
I have had 3 preemies, granted not super early but early nonetheless, I have no clear reason as to why, each time has been a different reason. I had a decent chance of a fullterm delivery when I decided on #4 but so far it is not looking like we will get near term. I do not have an incompetent cervix, I'm very healthy (don't smoke/drink alcohol, exercise regularly) and I had a pre conception meeting. I take all my pre natals and am fortunate to have good health insurance and access to very good specialists.
I have to say, I understand the longing for more children, its completely natural
Please try and follow the doctos suggestions though, even if they are not the cause, anything you can do to put yourself in the best of health can only be a plus.
On top of that there is your other kids, sb im sorry you may have to travel so far
We are lucky in that the NICU where we just moved is very close, but that aside there is the issue of taking care of the other children if you are on bedrest, in hospital etc. A good support network is invaluable, even so I can say firsthand its nigh on impossible to truely bedrest with other small children.
5 years is a long time, get your health on track and consult the specialists then come to a decision that works for your family. I wish you the best of luck, and wow 7 kids!