With sumup I get 0.5% cashback up to a whopping 10€/ month lol. There are other cards giving cashback, I think Amex blu has a 1% with no upper bound of monetary cashback

There are basically none, because the fees are low and card issuers are not allowed to rob retailers with insane fees. If you have a cafe or restaurant where margins are already low then it would be crazy to pay additional 2-3% tax on every payment.

No one wants to accept amex here.

My understanding is that they are much less generous on average compared to U.S.

E.g. for now I have the most premium card offered by my bank (50€/mo), and it gets me extra product insurance, rental car insurance, travel insurance, free lounge access, and some other things like that, but no cashback or similar.

I think some premium cards do offer cashback nowadays, but they are in the minority. Some cards also offer airline reward points. My experience is limited to Finland, though.

Presumably not very, because the large majority of Europeans do not use credit cards.

Normal cards not very generous. Fintech can sometimes give 1% cashback capped at 30-50-100 EUR.

Some weird crypto stuff can give more cashback, but it mostly for scheming nerds.

In the UK (which broadly follows the same rules), I get 0.25% with a Visa and 1.25% with Amex. Sometimes there are introductory offers for a few months.

Usually ~1%. Some direct cashback, some via rewards points (e.g. 1.5 AMEX MRP earned per EUR are roughly worth 1ct).

There are similar caps in the UK and most credit card rewards seem to be low after an initial offer period.

There are some specific discounts and benefits but not cash rewards.

It is illegal (not banks will not let you, actual legislation) here to charge more for card payments or discount for cash or bank transfer.