But if I use just Tor, then my ISP knows that I'm using Tor
Not necessarily, because you can use
bridges. These are Tor relays that aren't publicly known
I find this (Tor bridge nodes) a little unconvincing
the only way to keep the IP of the bridge node private is to never tell anyone about it (and so noone will ever use it). it only serves to perpetuate a cat-and-mouse game, where those with malign reasons to discover the IP of bridge nodes are chasing the newest nodes to censor them
supposedly there are pluggable transports for Tor that shape the traffic/packets to resemble generic HTTPS website traffic (which is apparently planned for BIP324 connections in Bitcoin, but the details are not yet available). If the relay then runs out of an IP range belonging to a commercial CDN, then connection fingerprinting is no longer possible. but this is "research level" Tor, the standard Tor client won't behave like that without modifying the config