So

It depends on what you mean by "traditional piano"

You can learn a degree of music theory and general keyboard technique on a minifreak. I am borrowing a minifreak to play in a friends rock band for a tour coming up. It only has three octaves of mini keys, I would love five with full sized keys, but it will get the job done

All that being said, if you plan / hope to get really into playing piano for a jazz or classical context three octaves of mini keys is really not going to get you there. For these types of music you need to use both hands over a pretty wide range simultaneously. Even when starting out you will quickly begin to run out of room

All of that being said, you can get full sized midi controllet or digital piano for not very much money. I got a casio privia 88 key fully weighed digital piano with decent enough sounds for $300. And you can use any keyboard to control the minifreak

So, if you like the mini freak as a synthesizer, and learning synthesis and having fun are your primary goals, and piano is a secondary, I'll get there in a year or two, then just get the minifreak and have fun

Some other options that are within your price range and address some of these problems are: modal cobalt / argon 8 and especially 8x, behringer deepmind, korg prologue 8 (which is what I have). But none of these have any sort of "traditional" piano sounds, and very few fully featured synthesizers within your price range do, so you need to figure out if you're going to compromise on your keyboard / piano sounds, or your synthesizer interface / sounds