Help with cross-domain conversion tracking in GA4

Hello PMMs. I need help setting up conversion tracking events for app downloads. So here's the context: There are two websites housed in different domains: (1) www.example.com, and (2) blog.example.com SEO work is being done on (2) while number (1) is our front and homepage. It's a mobile-first app. I'm trying to measure the conversion that SEO brings—with "conversion" here defined as app downloads from Google PlayStore. Now this homepage has the "Download from Google PlayStore" widget as a CTA on the above-the-fold area of the homepage. So, I want to set up conversions to know people who click through from our blog website (2), to our main homepage (1), and click the button to download our mobile app on Playstore. Here's the idea I've got it mind. ⚡ Use UTM parameters for every cross-domain links from (2) to (1) to record the "blog" as the traffic source on (1)'s GA4 dashboard. ⚡ Set up custom button_click event on (1) to track customers who click the widget to download our app. Constraints 🥲 If I set up tracking this way with the custom button_click event, how do I know that those clickers came through from the blog and then attribute that action accordingly? Is there a way to attribute this? 🥲 I have also heard that I need to set up cross-domain linking. Is this necessary for these platforms? The problem here is that both properties were created separately by the dev team before I joined and we've now collected a ton of data to start afresh. So measurement IDs are different. Please, I'd like advice.