Why not 'toch'?
Numbers can also be translated as "cijfers"
Why not "maar"?
Which is more common in casual native Dutch, "Echter, de nummers zijn niet goed," or, "Echter, zijn de nummers niet goed."
The former is used. You could also say "de nummers zijn echter niet goed".
Why not Alhoewel,...?
Alhoewel means although. Echter can be translated in however. The words have different meanings :)
In this context, however, the words 'however' and 'although' would have the same meaning in English.
Why isn't "echter" considered the first element in the sentence and therefore followed by the verb "zijn"? Sentences like this really make the "V2" order confusing!
Why not zijn geen goed?