Rather than making the weights heavier, you can make ST more challenging by doing it slower. (It sounds counter-intuitive, but if you do them slowly, then it is your muscles doing all the work, rather than 'bounce' or momentum. Take 3 seconds for the 'up' movement, and 3 seconds for the 'down'. Bonus points for holding the 'up' position for 2-3 seconds.
When you do purchase new weights, consider an adjustable set. While they are more expensive than a pair of fixed weight, they can grow with you, and are cheaper in the long run than a whole set of different fixed weights.
The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man.
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