Bards got their 6th level spells before wizards or clerics advanced to 7th level spells in ADnD 2nd edition.
They had fewer slots however. And they had to put a precious 15 into charisma...
But they were not so much behind as you might expect. Especially if you use the optional class based experience bonuses for the bard class. Although as a rogue they would get 2xp per gp found... Which is usally more than 200xp per spell level cast.
So relegating bards to 3/4 would be possible. But within 5e it would be ill advised.