I used the Tacx desktop app (Windows) during the nice months this year when I wasn't riding inside much to do basic workouts in ERG mode, and it works fine for the most part. It lacks some of the features (for ERG mode) of the paid apps like being able to adjust the power levels up / down depending on how you're feeling, or to skip an interval if you can't complete it. It's also pretty annoying to get ERG re-started after stopping since it doesn't add resistance and you have to spin like crazy to get the power up to the target before it re-engages.

I might also look into Golden Cheetah for next year when it's time to pause Zwift again. It seemed pretty hard to use the first time I tried it.

RGT Cycling is another free app that offers free ride mode, but no workouts in the free mode. I suspect like most free apps they will probably have to drop that eventually to be profitable.