There's no such thing as a midfield too lean. All you need structurally is 2 - 3 captaincy options, it's just the case that midfielders are usually the highest scorers, so you want a couple of premiums in the midfield to fulfil that role.
Whether going for this structure is worth it is another question. With quite a few rookies being available across both defence and midfield, I keep arriving at a 3 defence premium structure rather than 4, as I believe the midfielders have better value for money at the same price point.
E.g. Think from Berry to Rowell to Cripps to Neale to the ultra premiums, you would take the midfield premiums/rookies/mid-pricers over their defence counterparts.