Will table partitions help me?
Partitioning does not sound like a good fit for your scenario. Partitioning isn't something to be taken lightly. It requires a lot of planning and you'd have to rewrite 5 billion rows in order to convert the data. Instead you should consider a UNION ALL view. Depending on your requirements you could even make it a partitioned view.

A view will give you an easy way to query your data without any date movement. It's by far the simplest solution.

In general, partitioning doesn't improve query performance. However, with columnstore tables it can be helpful if it gives you better rowgroup elimination on the partitioned column. That can be accomplished without partitioning depending on how you build your columnstore tables. If you don't modify the old data it should be relatively straightforward to build your columnstore in a way that maintains order.

Columnstore tables do support partitioning, if you do have a legitimate need for it at some point. It's easy to experiment with it. Simply try to create an empty, partitioned columnstore table.