I suggest adding a system of guaranteed high-tier merges that scale with player level.
The higher the level, the better the guaranteed merge results.
For example:

Levels 1–10: 3 items = 1 guaranteed merge

Levels 11–20: 5 items = 2 guaranteed merges

Levels 21–30: 7 items = 3 guaranteed merges

Levels 31–40: 9 items = 4 guaranteed merges
and so on.

This would reward progression and make leveling up feel more impactful.