One other method would be to pick a voter and split their representation equally among the candidates that they have approved. This would be strategy-proof. However, it fails candidate Pareto efficiency.
1 voter: AB
1 voter: A
B would get 1/4 of the weight, and A 3/4. But B is Pareto dominated by A.
Another method is called the Conditional Utility Rule. This puts all the voter's representation onto the candidate that is most approved overall (or splits it equally if there's a tie). This guarantees a the maximum total approval score among a proportional result. But it fails IIB.
2 voters: A
1 voter: B
7 voters: AB
This would weight A:B 9:1. Passing IIB would give 2:1. But despite guaranteeing the maximum total approval score for a proportional result, it still fails multi-winner Pareto efficiency. It can sometimes be possible to find a set that dominates the winning set, although the result won't be proportional. See the result on this page. This is another reason why the multi-winner Pareto efficiency criterion is not necessarily a good thing within the voting election landscape.
I think this is largely it. This project hasn't purely been altruistic - it's been helpful to me by laying everything out for reworking my COWPEA paper!