I find it odd that party chairs make appointments. Why not have the president make appointments with a same-party rule?
That could easily result in someone party-switching to give one side an advantage. For example if I resigned and let's say Razze suddenly switched to DA then was appointed and switched back, then Labor-aligned senators would have 1 more than before.
We could have a rule where the appointee had to be in the party for a certain time before the creation of the vacancy or that the president has to pick someone from a list given by the party.
Still pretty easy to game the system if a party just stores 'plants' in the other parties for this express purpose. Best to just stick with the current system. If it ain't broke...