My counter argument: I use WhatsApp all the times and nothing I talk about on it ever shows up in my ads. A hefty amount of adblock may help here, as does the fact I live in the EU where the worst tracking is illegal.
Something on your phone is probably leaking data. Most suspect are third party keyboards, accessibility apps, apps with access to your photos and videos, or even Google Assistant. Third party keyboards can easily track what you're typing, accessibility apps can parse what you're saying or typing, and Google Assistant will take a screenshot of your current screen when you invoke it.
Other options are clipboard scanning (i.e. on older operating systems) and perhaps link preview services breaking out of e2e.
Finding what app is selling your information is difficult. For starts, you don't know which device is leaking. Ad companies are smart enough to see the connection between you and your wife. Her search results alone can probably make ads appear on your device!
Also consider the Baader-Meinhoff phenomenon. You can only track special topics if you track the topics of all ads and apply some statistical analysis. If you get blasted with ads all day, you'll notice the ones that you're on the lookout for. Pausing your scrolling through the app to take a screenshot will then reinforce the e-stalkers' algorithms.
If you have two old phones lying around, try repeating this trick with phones that are completely wiped, without any Google account logged in, with firewalls to block anything but WhatsApp from talking to the internet. I bet you'll find that those devices won't generate ads.
Why do I think that? For starters, enthusiasts decompile and analyse WhatsApp APK files all the time, in search for rumours and beta features to report about on tech news sites. If at some point WhatsApp added a secondary information channel about your messages (whose encryption is reasonably proven), reporters would've made a HUGE story out of it. A single line of decompiled code can send tech outlets into a frenzy of Meta accusations and let loose the EU's regulatory commissions for lying to customers. It'd be the scoop of the year!
Personally, I think "Google's keyboard or Instagram's gallery scanner is leaking my data" is a lot more likely than "WhatsApp has never been analysed enough to find the magic leaking code".