What did the spark plug look like that came out of the cylinder with the low compression? Was it oil fouled?
Also, where do you live and drive? Is your Prius driven in snow and ice, or hot desert heat (or both)?
What engine oil are you using?
The dealer should have checked the inside of the cylinder bore with a bore-scope; and they should know if the cylinder walls are scored. Was this done, or was it an extra expense and not performed? In any case, what did they recommend regarding repairs?
148,000 sounds really low for engine problems, especially on a Prius, when the ICE runs less than on a regular car.
I'd be tempted to find a long downhill grade and run the engine in "B" for several miles (downhill) to try and loosen a sticking ring. (Engine braking raises the engine vacuum and sucks more oil through the rings, which might break it loose, if that's the problem. However, this won't help; if you have worn valve guides.)
You didn't mention what part of the USA you live in; are there mountains nearby where this can be done? The long downgrade out of Yosemite, or a few roads in the Colorado Rockies might provide long and steep enough downhill runs to see if this might help.
Personally, I'd steer away from using STP, Marvel Mystery Oil and similar oil products. They usually cause more problems than they fix; especially in modern engines, where extra oil through the air intake can actually clog and/or ruin the mass airflow sensor and/or catalytic converter.
You may wish to consider having the dealer pull the cylinder to see what's going on. This can be expensive; and it puts you at the dealer's mercy if they decide you need to rebuild the engine. This should probably be a last resort, when you have already decided you want to keep this car even if it requires investing several thousand dollars for repairs.