Given the forms and curriculum you practice, I tend to think you'll find more familiarity aligning yourself with a senior Tang Soo Do teacher if it should come to that. There are plenty of credible TSD grandmasters who would likely be very happy to take in students of your GM, who is held in high regard.
Of course if belonging to the biggest, most world-wide TKD organization is a consideration, then we all know that path starts with the Taegeuks.
I am familiar with several 'orphaned' instructors in a number of systems. Most usually just stay at whatever their last rank was which is mostly adequate if they have reached something like 5th dan at least as it is a rare student who will reach the same heights. One guy just proclaimed his 5th dan the highest anyone could reach in his independent school. He has promoted his senior student to the same rank and they co-run the schools together. And a couple do something a little dubious in my opinion like have their students (local black belt association) promote them to a higher rank by acclamation.
My karate teacher is a 7th dan and he has no living teacher so he has been at 7th dan for years and years now. That also means I will never rise higher than 6th dan myself likely which I am OK with. But if my teacher passed away and I wanted to continue to achieve higher rank, I suppose I would need to find another teacher to accept me, which is no comfortable task at my experience level.