I sympathize with your dilemma. I am caring for some ferals outside my apt building. I know there is a strong possibility that at some point someone will turn me in and the apt will demand I stop. it would most likely be suburb and many have indoor only ordinances so even buying a house wouldn't necessarily allow me to relocate them. I have had no luck in several years of looking, finding a suitable country place and rentals wouldn't allow me to bring a bunch of ferals, even if they allowd "a pet".
If I were to find some place, do I relocate some of them? Abandon them and hope for the best?
I am not making any decisions till it happens but I have this underlying anxiousness about it all the time (people of this board are probably sick of hearing about it
I imagine if you have some shed or garage that you can keep her sequestered in for a couple of months till she 'forgets' about the old place that would probably help. At least you have a fairly long relationship and she knows you. That should help too. Are there still others?
I think after getting her settled the biggest worry might be other resident cats who will fight newcomers. At 15 she might not do very well with that. If you have the option of sequestering her for awhile you maybe can scope out any ferals in your new location and TNR, might help with the fighting.
Good luck