Clothing vertical, same promo window year-on-year, and it's tanking - hate that feeling. running PMAX + Search here too, normally rock-solid month-to-month, so a drop like this during a prime sales week screams something structural.
you mentioned turning on age restrictions (blocking 18-24 and 55+). that can crush volume if those segments were contributing any cheap conversions or high-intent clicks. even if your core buyer is middle-aged, you might have cut off a profitable fringe that was padding your CTR and lowering your blended CPA. I'd test opening those back up and see if the conv cost recovers.
domain change is the bigger suspect. It absolutely affects ad performance - even if you don't see a direct change in impressions/clicks. quality Score is tied to landing page experience, and a new domain means Google has to re-learn your relevance signals. That can silently tank your search impression share and raise your CPCs on PMAX, even if the raw clicks look similar. i've seen this happen with rebrands: same product, new URL, conversion rate halves for the first two weeks until the algorithm recalibrates.
your conversion rate went from roughly 1.6% down to under 0.7%. you're right that monthly conv rate can be misleading - I've had months at 0.3% with record revenue because of massive top-of-funnel volume. But a 60% drop during a high-intent event is a red flag that either the audience targeting is too restricted or the landing page trust signal (new domain) is killing the conversion flow.
check your search term reports for the Search campaigns - are you still getting exact-match impressions on the usual branded terms? and verify your PMAX asset group URLs all point to the new domain correctly. one broken URL can f*ck the whole campaign.
