As the above poster said, 9 years is a wonderful and old age for a bun, they were probably 9 years of wonderful times you both shared together.

I havent been brought similar, but I have lost buns before. Unfortunately I didn’t get to say goodbye as it was so sudden and you do feel like you could have done more, but I believe everyone knows its time and that was there’s. The pain still hurts 10 years on but I look back and smile at the pictures and memories we have (and the occasional scar from a grumpy bite)

I know its hard but I think you did the right thing, I know this may not hold weight coming from a stranger but you can’t hold regret over this, the vets thought it was the right thing to do at the time as so did you, that tells me your gut instinct new it was time and probably so did Frank, it’s always hard to let them go, but having you by his side was probably the best way to go, especially because you got to say goodbye one last time.
Keep Frank’s memory in your heart and remember the wonderful years you shared together.