I’m sorry you found the initial material disappointing. The early material is, of course, speculative. If you’d like to put your feet on more solid ground, could I suggest moving immediately on to no. 6, ‘The Sophists and the Polis’?
In fact, while there is some merit in following any history chronologically, the various major segments (in particular, those on individual thinkers) are relatively stand-alone and so can be taken in the order in which they are of most interest to you.
I hope you find the material from the Sophists onwards more acceptable.
With every good wish,