1. 'which' is not really correct here, it should be 'that'
2. You can remove that, but you need to be clearer in your explanation (the second part), and
3. You need 'the' to reference a specific goal you had..
so, you should say '...instead of the 0.75kg (that) I'd planned to lose'
However, you should really emphasize that you did better than you planned, so..
'... much more than the 0.75kg (that) I'd planned to lose'