Perhaps Zwift? It has lots of workouts available for you, some that are just one-offs and some that are in a series.

I'd recommend Trainer Road paired with one of the cycling platforms -- you can do the workouts while "riding" on group rides or solo on a virtual cycling platform, e.g. Zwift (pay), IndieVelo (free), or MyWhoosh (free.) Zwift has more "carrots" and many, many more people and groups than other platforms.

I use IndieVelo, because I tired of Zwift's worlds and my favorite type of group rides went away. I found the routes, worlds, and real people quite engaging for a long time. I suspect you'd find Zwift more engaging than other platforms. Zwift has training plans for all sorts of goals.

IndieVelo is mostly bots, but there are some real people, and the bots are more like real riders than on other platforms. The interface is much more intuitive and easy to use than MyWhoosh (horrible interface) or Zwift.

The nice thing about Trainer Road is that it will give you a plan and change the plan and workouts as you go along. That auto-customizing felt good to me, and I made some fitness gains. Using Training Peaks requires that you add a training plan from someplace -- a coach or some free one you find. I didn't use a coach and felt like it was dull and clumsy. Other people love it.