@alex_pisa
Hi Alex
I think there's likely two issues here - the SEO one, and the PPC/Ads one.
Hi Alex
Google Ads doesn't particularly care about duplication or indexing, so you can have variants for this purpose that are simply noindex and not linked to on your site. Of course it's also find to use pages that exist as international SEO variants, but it's not necessary to do so.
So, for the PPC issue - I'm not an expert, but my understanding is that landing page experience will affect your cost. However, it isn't related to "duplicate without user-selected canonical" - instead, it's a separate algorithm that figures out whether the ad and the page are relevant to each other. A PPC consultant would be the right person to talk to this about.
For the SEO issue - it sounds like Google maybe isn't respecting your hreflang tags, for it to be flagging them as duplicates. This could be because their content is too similar - even if it's in the same language, it should have localisations for Google to respect it as meaningfully different. Alternatively, it could be because the markup is incomplete.
If you do decide to use canonical tags, each localised page should canonical to itself, whilst keeping the existing hreflang markup pointing to translated versions. Canonical tags can be a good way of sweeping up any variants within a localisation (e.g. UTM tags).
Hope that helps!
Tom