I don't think this is an issue with the on-site analytics, but with the quality of the traffic and website performance. If you make sure that:
1) Your site loads in under 3-4 seconds for any user.
2) The user is interested enough to wait 3-4 seconds until the page loads.
Then most issues will be solved.
The problem with ads in many cases is that the traffic they send is of very low quality or just bots. In the end you already know from your ads provider how many users they say they sent and you should always use that when calculating ad conversion rate.
Also note that using Cloudflare will count as bounced users who never actually even tried to load your page (bots, crawlers, scrapers, all HTTP requests).
1) Your site loads in under 3-4 seconds for any user.
2) The user is interested enough to wait 3-4 seconds until the page loads.
Then most issues will be solved.
The problem with ads in many cases is that the traffic they send is of very low quality or just bots. In the end you already know from your ads provider how many users they say they sent and you should always use that when calculating ad conversion rate.
Also note that using Cloudflare will count as bounced users who never actually even tried to load your page (bots, crawlers, scrapers, all HTTP requests).