These are very different systems.
Scylla is a high-performance C++ version of Cassandra. Scylla/Cassandra are wide-column databases, or better referred to as advanced key/value databases. The original research is from Amazon’s Dynamo white paper and it’s similar to Amazon DynamoDB, Google’s BigTable, Apache HBase, Microsoft Azure Table Storage, and several other systems.
Scylla/Cassandra has consistency levels that can be changed per-query, for eventual to strong consistency, but it is designed as a high-speed, write-heavy, multi-region, highly available, distributed sorted key-value store. It might look like a relational database with SQL and tables but it acts completely different internally.
It has tables with a primary key, where the entire row is stored as 1 big chunk that’s accessed by the key, and each column within that row is another key/value pair, which is why it’s better called a nested key/value store. The primary key is the only way to access data and get a row, or a prefix of the key to scan rows. Scylla/Cassandra support CQL which stands for the Cassandra Query Language. It looks like SQL but is very minimal and just an easy way to insert, update, and delete data by key.
Scylla does not support joins, there are no full secondary indexes (although you can create your own by just writing duplicate data), updates and inserts are the same (everything is an upsert), there is TTL and table compaction, anything column can be null and schema is very lightly enforced, and there are no aggregations or CTEs or any other advanced queries.
CockroachDB is a full-fledged relational database designed for automated sharding and replication of your data. It won’t match the pure throughput and performance of Scylla but it is designed for much higher level querying and general usability. Unless you really need Scylla and the specific data model it has (which doesn’t seem so from your question), then I would recommend sticking with CockroachDB.