I think Andre will be good in Bundesliga and seal a Premier League move within 2 years, Chelsea or Arsenal. If Mendes works his magic, it might be a Barcelona move.
Andre Silva has the talent, but not the fighting mentality, as you mentioned. In a league like Bundesliga, usually only the talent is enough to succeed. E.g an average player like Paco Alcacer is scoring every week. For another instance, Rodriguez and Hakan looked like some demigods for years over there.
Frankfurt played some good football last year, very good, quick and combinations in attack. As long as they feed AS the same way they fed Jovic, I don't see why the former shouldn't score more than 15 Bundesliga goals and 5+ in Europa League.