Combining the answers from @Ortomala Lokni and @rogerdpack, another alternative is to have the dependent service monitor when the first one has started / done the thing you're waiting for.
For example, here's how I am making the fail2ban service wait for Docker to open port 443 (so that fail2ban's iptables entries take priority over Docker's):
[Service]
ExecStartPre=/bin/bash -c '(while ! nc -z -v -w1 localhost 443 > /dev/null; do echo "Waiting for port 443 to open..."; sleep 2; done); sleep 2'
Simply replace nc -z -v -w1 localhost 443 with a command that fails (non-zero exit code) while the first service is starting and succeeds once it is up.
For the Cassandra case, the ideal would be a command that only returns 0 when the cluster is available.