Conversion tracking with Google Ads on Thank-you pages
We are doing this exact thing at the moment. I was kindly given some code that sends events to GA and Facebook pixel on the Thank You page by @jtwyman on Mattermost. A Civi developer worked with us on it to build an extension - and evolved it slightly to respect DO NOT TRACK.

We haven't got it working 100% yet. We have an issue that completed donation £values are not being sent back to GA (membership values are being sent correctly). It's in our Github repo but private at the moment until the issue is fixed, but happy to make public and share when it's working correctly.  

I'm not sure how others are handling this.

EDIT: After Googling, I found that Matt Wire released an extension at the same time we had ours developed:

https://civicrm.org/extensions/google-analytics-event-and-e-commerce-tracking

I've raised an issue asking to have our Facebook Pixel and DNT features added so that we can just star using that rather than maintaing our own:

https://lab.civicrm.org/extensions/webtracking/issues/2