What port numbers above 1024 should I not use as an alternative SSH port?
You can use any port number above 1024, but you may find a few problems if you use a port that's also a dedicated alternative port for a different service you are using. This includes 8080 (TCP/IP), 8888 (web development), and 27017 (default MongoDB port). Alternatively, 22000 is an alternative port for SSH services. Just don't pick a number that's too high, since some ISPs and WiFi routers have limits on these.