Ignore the volca fm imo, fm without velocity sensitivity.. the expressiveness of fm via velocity is one of fm’s strongpoints and they left that out to a large extent. Weird!!

tx18z, despite being 4 op, in some ways has more features and possibilities than tx7 but also vice versa, so try reading into those and pick which one is best for you, either is a nice buy in terms of what you are looking for.