why do I feel so detached from my parents

Yet your final sentence is telling in that you say they would never knowingly hurt you. Can you not forgive them about the school bullying, it wasn’t the issue 50 years ago that it is now. Parents didn’t go rushing off to school to complain then. I was bullied so I know.
If your siblings don’t understand how you feel, why not talk things through with a friend or maybe your adult children. It might help.

It sounds as if you are the grown up, you have family and many friends and a life of your own - well apart from your parents. So you have moved on and maybe with the Christmas season on the horizon and the problems there have been due to lockdown you are now feeling pressured to act in a way which is difficult for you - if not impossible.

Your life is probably fine now, but this wasn't always the case. Maybe you have blamed your parents for past problems and have accepted that they have not supported you as they should. Rather too late now, I would think, and quite cruel to be confronting them about stuff at the end of their lives and especially when you seem to have a good life now, brought about solely by your own endeavours. Possibly you do have some concern about your relationship with them but you don't want to be coerced into behaving in a certain way which is only to make your siblings feel better. It's not your job to make them feel better. These are your thoughts and feelings and you shouldn't feel guilty or bad about how you feel at this time. You should behave in a way which makes you feel comfortable so that you don't feel strained or awkward around your parents.

So now perhaps you should decide...let it go, or do something to change how you feel if you can. Then finally you can stop concerning yourself about your own feelings with regard to your parents and maybe manage the odd visit or time you spend with them during their last years without feeling conflicted.

How sad that you feel this way. There's no handbook for parents. Most of us will, at some point, look back and wonder whether we did right by our children. I have grown up children of my own and on one or two occasions, have been pulled up on my parenting skills. Although I laughed it off at the time, it did hurt.
I am not a psychologist, but I am quite sure your parents will be devastated if you mentioned your feelings to them. They are in their late eighties, and surely, you don't want them to be riddled with guilt in their late years.
I can understand your feeling of being 'let down ' by them ...perhaps they felt if they ignored the issue, it would go away....burying their heads in the sand, comes to mind.
Please, please see them at Christmas. Best wishes ..

It is perfectly normal to create your own life, totally different from your parents.

When was the last time you celebrated xmas with them?
If it has been a long time, it is normal that you have built your own traditions and are perfectly content with how your life is setup.

Now, add the fact that you were not protected when bullied and not believed by your parents when you needed them - a case of "they made their bed now they lay down on it"

Why shall you have to move on and be a grow up? You are already a grownup.
Are you been told - "be the bigger person" by your relatives?