I have a small Flutter app and plan to promote it through both posts and ads on Google, Meta, and TikTok. The customer journey follows one of these paths:

• Ad/Post or anic traffic → Website → Click "Install app" → Redirect to App Store/Play Store.
• Ad/Post → Direct app install → Redirect to App Store/Play Store.

I already track "Install app" clicks on my website via Google Analytics but lose visibility once users are redirected to the app stores. I need a way to:

• Attribute app installs to the correct platform (Google, Meta, TikTok) for campaign optimization.
• Measure conversion rates & ROAS to improve ad efficiency.
• Unify analytics across web and app in Google Analytics for a complete picture.

Possible Approaches:

1. Use a Mobile Measurement Partner (MMP) like AppsFlyer, Branch, Adjust, or Singular. However, I have a limited budget and it seems as if these services can get expensive.

2. Integrate platform-specific SDKs such as the flutter_meta_sdk in the app. But this only covers Meta (couldn’t find Google or TikTok sdk packages for Flutter). Also, how would such an sdk in the app know whether an app install originated from say a Meta post / ad and not from a different source? I'm guessing that because the user will be seeing the ads / posts on their mobile, there is a tracking of the device id to make the correct attribution.

3. Manual setup using attribution parameters:

• On the website:
  • If the traffic has arrived from a post / ad, capture the platform click ID identifier (fbclid, gclid, dclid, ttclid) from the url utms. Note: if it is a direct ad/post -> app install (i.e. no website visit in between), I presume a click id will be passed in automatically.
  • Use dynamic links for the “Install app” button and pass in that click ID. (Firebase Dynamic Links seems to have been recommended to use but it is being deprecated.)
• Upon app installation, retrieve the click ID.
• Send the install event with the click ID to Google Tag Manager server-side (GTM-SS) which is set up to forward that install event to the platform associated with that click ID, ensuring correct attribution.

My Questions:

What setup do developers with limited budgets typically use to achieve the above, i.e. to:

a) Track and attribute app installs to platforms correctly?

b) Get a unified view of web and app analytics in Google Analytics?

Would love to hear about effective setups that don’t require a large budget.