The right style is everything in Biab. Try finding Graham (Furry) he knows about this style of accordian playing and if he's not around, try Googling "Band in a Box styles".
The best way to do it and probably what you will wind up doing, is to jump right into the Stylemaker and make your own. That how the pro's do it but it's a bit daunting for somebody who's just learning the program. I'm pretty sure you're going to be using strictly midi for now because I don't think there are Real Tracks that will work for what you want but I could be wrong about that since I'm not that familiar with this stuff. Like Mac said, experiment. The big difference between midi and the Real Tracks is the RT's are prerecorded and you can't change them. As you've already found out, if that bass part isn't exactly what you wanted you're stuck with it. Midi can be tweaked to do exactly what you want it to do but you're playing a synth as opposed to hearing studio recordings. Such is life in the world of computer music.
In my opinion you don't need Real Band yet but keep it in mind for the future. You can also Google some song names with the word "midi". You may find some midi files of exactly what you want. Those are different from Biab styles. If you find those, then Real Band is better so come back and ask about that.
Most likely, as a brand new person to these programs Biab is enough for you right now. I also use Jamstix, it's way beyond what you need at the moment but we can talk about it later.
With the right style, Biab will sound great assuming you have a good sounding synth to play it through.

Bob

Biab/RB latest build, Win 11 Pro, Ryzen 5 5600 G, 512 Gig SSD, 16 Gigs Ram, Steinberg UR22 MkII, Roland Sonic Cell, Kurzweil PC3, Hammond SK1, Korg PA3XPro, Garritan JABB, Hypercanvas, Sampletank 3, more.