Besides Celtic, Hearts (this season) and Rangers the Premiership is mostly EFL Championship quality. Celtic and Rangers would need a miracle to even touch European football if they got their wish and joined the Premier League. Celtic's Champions League qualification via the playoffs (which they didn't even get this season since they lost to Qairat from Kazakhstan) and their otherwise high profile for multiple reasons (fan culture, domestic trophies, etc) has elevated profile of the Premiership. I really hope Hearts do shake things up this year (same goes for Lens in Ligue 1) just to see a different winner for once.
You mentioned Belgium and you're right, the Belgian Pro League is better than the Scottish Premiership. While rankings are often misleading, Belgium is ranked eighth in Europe in terms of UEFA coefficient, while Scotland is now 18th (down from 14th). In between Belgium and Scotland in the rankings is Turkey, the Czech Republic, Poland, Greece, Denmark, Norway, Cyprus, Switzerland and Austria.