There are some nice fresh materials published by Chess in Schools and Communities: https://www.chessinschools.co.uk/chess-at-home
Chess Tips
| MIDDLEGAME TIPS: |
Decentralising is always suspicious, particularly for knights, but also for long-range pieces like Queens. This is a corollary of Stean's assertion that the most important feature of any position is the activity of the pieces, since pieces are most active in the centre. |