You can certainly go higher than a terabyte - 4 or so is common, Ive heard of people doing up to 12 tb with the awareness that time to replace scales with size on disk, so a very large host will take longer to rebuild than a small host

The 50% free guidance only applies to size tiered compaction, and given your throughput you may prefer leveled compaction anyway. With leveled you should target 30% free for compaction and repair

You don’t need more than one Cassandra instance per host for 4tb but you may want to consider it for more than that - multiple instances are especially useful if you have multiple (lots of) disks and are running Cassandra before CASSANDRA-6696 (which made jbod safer).

Jeff Jirsa