I'd say the best thing to compare your structure to for inspiration would be something like One Piece.
In One Piece, a large crew of pirates sail around the world on a ship. They have a shared goal and to get to it, they need to visit different islands along the way. Each Island is an almost self-contained adventure with its own villain, supporting characters and a heroic climax, and often a particular island can serve as a place to meet a new crewmember, or an arc that explores one of the existing ones more deeply by being tied to their past in some way or by having a conflict that's close to their heart.
I'd structure it so that getting soul 1 is the most straightforward, like everything is where it "should be", there's a tough challenge to get it, but there's no twist on the formula. This means I'd suggest putting the first one as the soul for the knight with the lowest self-confidence, and the barrier to getting it is largely "can I overcome this challenge?" and the resolution is either "yes, with the help of my friends" OR "yes, and I did it alone, I'm not the useless member of the team who has to rely on all the others after all!"
But then after soul 1, they all tend to have some kind of TWIST. So like... the place where they need to get to has caved in and now it's at the bottom of a pit, or a nest of monsters have moved in, or the instructions to the challenges have been lost, or thieves have stolen the thing they want and taken it to their airship base, or the place the soul resides is now at the centre of some huge Hunger Games like battle royale currently ongoing... you know, something like that.
The twist should be in some way tied to the skills and the personality and flaws of the character that soul is relevant to, and their victory should highlight why we should love that character and think they're cool. There should always be a moment where that knight realises: "Oh! I'm the only one who can do this. Leave it to me!"
Around the 4th or 5th one, the twist should be that the big bad or their second in command involves themselves directly to sabotage the team's efforts, to remind the audience of the looming threat.
So my suggestion in summary: Each soul quest should be a distinct mini-arc with a different environment and a different character focus that leaves that character and their relationships to the team a little different from how they were before.