Playing small number of openings is a general advice by IM/GM teachers for beginner players. This makes sense since the focus is on endgames and middlegames in the initial learning curve.
There's nothing stopping you from practicing tactics even if you play d4 openings as white. I have been a mainly d4 player as white so far (London, BDG, QG) and have had quick ratings growth (Start -> 1650 chess.com rapid in 5 months).
Also, an opening is more like 7-10 moves (not 2-3 moves) where all the minor pieces have been developed and the player has castled.
There's nothing stopping you from practicing tactics even if you play d4 openings as white. I have been a mainly d4 player as white so far (London, BDG, QG) and have had quick ratings growth (Start -> 1650 chess.com rapid in 5 months).
Also, an opening is more like 7-10 moves (not 2-3 moves) where all the minor pieces have been developed and the player has castled.