Alternative Data Structure to DataTable

Depending on your use case I would recommend using List<object[]> (since you mentioned dynamic schema) as central data structure, but you will need to maintain the schema info yourself if you need it later on.

If you need to bind the UI to the data this approach will add a lot of extra manual work, it's better suited for background processing of large amounts of data.

We have used this approach in the past and were able to save 2/3 of memory and 80% of execution time when bulk handling data compared to data tables.

One alternative way of approaching problems like this: use a sqlite database in memory.

Sounds like a weird thing to do at first, but you can put quite complex structures into tables, and you get the whole power of SQL to work on your data. SQLite is a tiny lib, so it won't bloat up your code. Integrating the DB into your code might be kinda strange at first, put performance should work on huge data sets (since that's what DBs are made for). And if you ever need to save that data to disk, you are already done.

Depending on the details of your problem, it might even be a good idea to move to a bigger db back end (e.g. postgres), but that is hard to tell from here. Just don't dismiss this idea too easily.