@Wim VD The reason the chord tone approach sounds better to you is because you are using the harmonic framework of the progression. Chord tones are the safe, sweet notes the chords or arpeggios are made of. They are the notes you can hold or land on. When it comes to the seventh chords, the third and the seventh are the guide tones and you should target one of them when making the chord changes. Of course, the chord tones alone don’t make a great solo, but if you start with them, you will get the harmony into your ear and you will hear the chord changes. Then, you can start adding approach notes and here the devil is in the details that are way beyond the scope of a simple post.
So you decided to focus on learning the arpeggios starting with the dominant 7th chords. That’s a good idea because all you need for a traditional 12 bar blues is this one type of chord. The harmonic simplicity and structural economy of the 12 bar blues makes it the perfect vehicle for tackling soloing and improvisation in the early stages. So again, you start with chord tones only, then you add approach notes thereby building your solo from the bottom up. You can learn the arpeggios in isolation or from the perspective of its parent scale. The mixolydian which can be thought of as a major scale with a lowered seventh is the parent scale for a dominant 7th chord.
After you’ve completed Session 19, you may want to reach for a resource that thoroughly deals with soloing and improvisation in your chosen genre.