If you want to use ERG mode, than those all sound like good options.
One cool thing about doing intervals while "free riding" in Zwift is that it can be fun and interactive, almost like riding outside. There are segments in each map that are timed and have rolling leaderboards with whomever is logged on at the moment, which can be more than a few thousand. So, just like you would IRL, you can cater the duration and specificity of your intervals based on whatever segments are available, whether it's flat or up a hill (high resistance). It is fun and motivating to go for segments sometimes, especially if you get a powerup.
Tomorrow morning for example, I'm considering doing a zwift race, than tooling around in Watopia for a while before going for Fuego Flats reverse on the TT bike. Last time I did that I got the fastest time and got to wear the Fuego jersey for a while
