Start breadcrumb from specific page

I would like to implement a breacrumb navigation which shows only the subpages of a certain second level page. What would be the best way to do this? I would like to make it kind of future proof, also if the page will be moved to some other place in the tree later on. So it would be awesome if I could somehow specify breadcrumbs from page “xyz” on.

You would need something that securely identifies the page in this case (template, ID) and you would probably have to search in the complete $site->index() which could become a problem if the site has many pages/files.

Identifying by template should work in that case. But the amount of pages makes me a bit unsure.
Can you think of an alternative way to get to the desired result?

How many pages are we talking here?

Could you elaborate on your use case?

And what are possible locations for this page, anywhere in the tree?

Currently around 100, but probably growing more and more.
We implemented a new helpsection area. In there we would like to display the breadcrumb navigation.
But since we dont use it anywhere else, we also dont want to show the other parent sites in the breadcrumb.

The page is currently on the second level in the tree and will move to the first anytime soon. But even if it will be on the first level, we still would not want to show the home page in the tree.