Start by separating impressions from clicks before deciding it is an update at all. In the Performance report compare two equal-length date ranges and look at impressions, clicks, CTR and average position together, per query. Impressions flat or up with position held and clicks down is a SERP layout story. Impressions down with position held is a different problem, you are matching fewer queries.
The trap with stable rankings is that average position hides where on the page you actually are. A page can hold position four while position four moves below an AI Overview, a video block and an expanded People Also Ask. The number does not move, the pixels do, and CTR falls off a cliff. So read CTR per query rather than position.
Then split the affected queries by intent, and also by device and country. If the informational ones lost clicks while the commercial ones held, that is the answer-box pattern rather than anything you did. A drop that lives almost entirely in mobile, or almost entirely in one market, is a layout change on their end.
Technical last, because it does not fit your symptoms well. A crawl or rendering fault usually shows up as impressions falling, not clicks. What I would do instead is pin the exact day the decline starts and put it next to your own deploy history. Plenty of drops blamed on an update turned out to be a template change or a plugin update that went out the same week. Do you have a clean start date, or was it a gradual slide?
