I am going to give you the definitive answer to your question. Along with playing accordion professionally, I have been a professional live sound engineer for over 25 years and own a small sound reinforcement company -
www.authentic-audio.com
First rule - Buy once, cry once. Do not buy something that you will soon need to replace because your initial purchase was not satisfactory.
Buy the correct equipment the first time.
debra made an excellent suggestion in the Bose L1 system. There are other manufacturers that make similar design systems that are also high quality such as Electrovoice, dbtechnologies, RCF. These column systems will be far superior for your needs compared to a traditional guitar or keyboard amp.
I personally use a K-Array KB1 which is Italian made but unfortunately no longer being manufactured.
Portable line array for musician sound
www.k-array.com
If you are performing live, forget about stereo as very few people will be in the sweet spot. The people on far left or right won't hear a thing coming from the opposite side speaker.
Best accordion pick up system I have heard, hands-down, no comparison, is Harmonik. These start at around $500. Incredible natural sound.
https://petosa.com/collections/accordion-amplification-microphone-pickups
I see you are in San Francisco area. What is the name of your band. I'm in the North Bay in Sonoma County.