If you renew with the bank that you had your mortgage before, I would assume that they are a bit less strict on the affordability requirements. In my case, I never had a mortgage in Switzerland.
FIRE people are still considered an odd case (how dare you retire before the reference retirement age…) so the closer your are to reference retirement age, the less your are an odd ball to the banks. Meaning if you are 10 years away when you FIRE, I would definitely choose a 10-year fixed mortgage to avoid all the hassle. If you are further away, it becomes a trade-off between increased interest rate vs. risk of not being able to renew.