I think the concern is that you could still, in principle, draw districts that gave you lots of seats, because they're relatively small. STV isn't proportional by design. You want as many of their votes as possible to wind up in the fourth of three quotas, and as many of yours as possible to end up in the three quotas that win seats. For instance, it may be that Us in some state just suddenly start winning two out of three seats in most districts. Make sure the two seat districts go in areas where They have a bit more than fifty percent but not a full two quotas so they split one apiece. The goal is still to waste as many of Their votes as possible to get fifty percent plus one seats.

PR is no panacea. To avoid gerrymandering, you need an adequately high district magnitude. After all, majority-preferential is just quota-preferential with a magnitude of 1. And FPTP is just List PR with a magnitude of 1.