30 minute cycling plans using a power meter?

Last year when I was coming back from an injury (broken elbow) I used the Beginner Cycling Program (free on this website) for a while. They start at 30 minutes (which was about all my arm could tolerate at first) and work up slowly from there.The workouts aren't necessarily power-based but they do mention intensity levels and you can adapt them if you have a guess as to your current FTP. (Mine was about 200 but after several weeks of forced inactivity before and after surgery, I put it back to 160 and then slowly progressed from there.) The workouts are more frequent than 3X a week (I think 5 or 6) but again, you could adapt them and just work more slowly through the progression. The program says "Beginner" but as a 50 year old woman my size (about 115 pounds) with an FTP of 200, it should be clear I'm NOT exactly a beginner (and cycling's my weakness). But I thought it was quite good for rehab and rebuilding fitness.

Or just do something more generic like 10 minutes warmup, 15 minutes at 70-75% alternating higher cadence/smaller gearing and lower cadence/bigger gear, 5 minutes cool down. I did a that at times if a particular workout didn't suit me. For a while I was unable to do big-gear riding on my own trainer due to the weird position I was using to keep weight off my arm, and I couldn't always get to a gym stationary to do those workouts.

3x30min of cycling is not going to bring much bike fitness
However if it's all you have, warm up for 10-15min and go the rest at 80% of FTP at 60 cadence. It will build strength. The low cadence and that specific power will mean very specific force that will recruit specific muscle fibre that will build strength.

I can see that.

30min is probably the time you need to do short intervals in a high intensity workout. For example 15x1min on, 1min off.
Yesterday I did 6x(3min on, 2min off) which is exact 30min of interval work. But I needed 20min warm up and 10min cool down.

At one point the value of a workout is mostly "load" which is time*intensity. If time is too short intensity has to be very high. Very high for a while is OK, will not be great long term.

To properly do "very high" you need a decent warm up and at least a bit of cool down.