You've nailed the core problem, but here's the thing-without knowing what's actually shifted in your KPIs, you're just guessing at ghosts. Let me break this down the way I'd map a customer journey flow.
First, isolate two variables: CTR and conversion rate. CTR tells you if the ads are working (triggers in the funnel). Conversion rate tells you if the landing pages or site are doing their job (the decision node). If CTR is stable but conversion rate has tanked, that's a website issue-pure and simple. something changed in the user's path between clicking and converting. Could be the page layout, a broken form, even a change in copy that the dev team pushed without telling you. Happens all the time.
If CTR has dropped, then the problem is upstream-ad creative, targeting, or market saturation. That's a different logic branch entirely.
Once you've identified the bottleneck, run controlled tests. For a conversion rate drop, check if the site itself has shifted. If not, test offers: lead form vs. short video vs. one-click demo booking. See which node gets the highest engagement.
Also, don't ignore the market landscape. A competitor could have launched a cheaper alternative or an AI tool that automates what you used to consult on. that changes the entire segment's behaviour overnight. Do some actual research, not just dashboard staring.
Track the triggers. map the journey. The data will tell you where the friction is.