There are a few settings, that could dramatically change the results of your Apple Search Ads SKAG campaigns.
CPA Goal at the Ad group level
Match type at the Keyword level - Broad and Exact.
Different variations and combinations of these settings could provide shockingly different results, and we recommend you try them all.
CPA Goal is an underestimated feature, that could instantly reduce avg CPA of your campaigns. Try to start with CPA Goal about 80% of your target ARPU.
Campaigns with CPA Goal could take more time to have impressions and installs.
Broad match could help you to have much more installs, but the ROI results also could be worse.
If you have some specific Custom Product Pages for specific keywords, you can only use Exact match, otherwise, it will be irrelevant and all mixed together.