What are some reliable and trustable CPI install ads we can invest in? (For Android app)

Currently, we are solely invest in Google Install Ads, to drive real user install.

However, these few months, we have more money than we are able to spend.

We prefer to spend in faster rate, to drive up install rate faster.

The CPI of Google Install Ads is good. We are not however willing to increase the CPI, as we feel it doesn't worth the cost.

We would like to spend in other CPI install ads network. However, we can rarely find a reliable and trustable one.

We have tried Facebook Ads. However, its CPI is way too high, compare to Google Install Ads.

We try google for "cpi network to promote android app". However, most of the companies look pretty shady. They do not have a clear contact address, team member information, or introduction on how they are going to promote your app.

I was wondering, have you tried some reliable and trustable CPI install ads (Besides Google Install Ads), which you are happy to recommend? Thank you.