Hi Adrien! This is a good question - you can think of variables sort of like environment variables; they are always strings (so don't have an inherent type other than String) and are typically single valued. Blocks, on the other hand, are generally typed with rich schemas.
If you are using a string block you can continue to do so, but I highly recommend migrating that to a Variable instead - I think it will be less heavy than a block for you