I've got a website with a depth of 3 levels:

Home > 30 categories > 2,000 subcategories > 400,000 products

Some months ago (https://www.webmasterworld.com/google/5094171.htm) I was querying here about the 'excessive DOM size' of some of the subcategories, that show hundreds of links. It is causing some indexing problems (URLs of some products are not indexed).

You told me to think on the users, and to break those massive lists up into something more manageable. And I've decided to implement the following structure for each one of the 2,000 subcategories, regardless of whether they have 40 or 7,000 links:

'https://example.com/category_name/subcategory_name/' will show:

1) Firstly, an Ajax accordion with all the links, divided into sliced of 40 URLs, and
2) Then, links to the first 40 URLs (sorted alphabetically)

'https://example.com/category_name/subcategory_name/?page=2' and so on will show:

1) The rest of the links (sorted alphabetically), starting in the position 41

As far as I've researched:

Googlebot crawls and indexes Ajax accordion content

The infinite Scroll is one of the recommended methods for pagination (https://developers.google.com/search/docs/specialty/ecommerce/pagination-and-incremental-page-loading). I will follow Google's guidelines: [developers.google.com...]

As a consequence, I will:

help users with the Ajax Accordion

help Googlebot (along with the already-existing Sitemaps file of the 400,000 products)

avoid the 'excessive DOM size' issue

I would appreciate feedback and similar experiences.