That's right. If you want to be able to play in all keys on one instrument, a keyboard- or a chromatic button accordion is your choice.
The single row accordion is designed for volume and rhythm, and played very much "on the bellow". But it's not suited for French musette accordion, which is very melodious.
I have seen some very good diatonic musette players on Youtube (wow- but very difficult to play), but that requires a special instrument with a lot of extra basses. If you already have experience in playing on a keyboard, I would go for a keyboard accordion.
I play a very small one , so I can put more swing in my playing if I want.
See Link #3 which leads you to a piece of "French Cafe music" (which I call Musette). Not at all impeccably played, but it gives a good example of the possibilities.
Check out the original Nany, and there is also a very very good diatonic version of it on YT (much better than mine).
The original was played on a big chromatic button accordion by - I guess- French accordionist Gus Viseur, and was composed in the forties by Joe Colombo.
Of Slavic music I don' t know much, but they also seem to play big chromatic accordions, and not small diatonic ones!