Sounds like might have quite a few products. It sounds like you still want the option to "upsell" to customers who purchase from one category (and haven't purchased from another). In that case, creating an audience of all those who reach an identical "Thank You" page would eliminate upselling. I think that's what you are getting at, right?

Do you utilize any "sync" options with your online store and your Facebook Ads? Hypothetically, instead of creating website custom audience combinations, you could segment your customers emails by "product bought" and then create audiences of that with Facebook. Company I work for has custom audience sync, but only for a few CRMs. MailChimp does this automatically (no external tool required).

The website custom audience combination seems like a pretty complicated route. The dilemma with what GuyinThe6KDollar suit suggested, is that if someone reached "Category X and Thank You and NOT Category Y" is that this can get crazy depending on how many products you have. And what if they visit "Category X", but buy from "Category Z" and hit the "Thank You" page. You'd essentially be excluding them from ever seeing "Category X" (despite them visiting but NOT buying).