It's true that an InDesign document can have just one index, but you can get around that. For instance, mark up the index as normal, then mark up the table of cases as a single topic in the index and split it off after generating it. Or prefix all table-of-cases entries with a certain symbol, then extract it after generating it (and remove that symbol). That's in a single document. In a book (indb) you could use Eugene's suggestion of doing the index and the tables in separate documents.
In this way you can create other tables as well. I've used this method in several legal texts to do tables of cases, authority, and citations, and an index. Using text anchors isn't really an option, much too cumbersome.
InDesign's index is very limited, but you can do a lot with scripting (as Eugene mentioned) so that it becomes even useful. See https://creativepro.com/files/kahrel/indesign/lists_indexes.html and https://creativepro.com/files/kahrel/indesign/index-fixes.html for examples. See also https://www.kerntiff.co.uk/products-4-indesign/indexutilities for various index scripts.
You say that you started using InDesign only recently and don't know (yet) how to script. Your options are to do the job manually -- which, as you said, is very tedious -- or have a script written for you. It's not trivial so be prepared to pay a fee.
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