I think you’ve summed up Curve perfectly.
I did use Curve to exploit the cashback (both Curve cash, and the underlying credit card), but once that had finished, I could see no point in keeping it.
Google pay has all of my compatible cards in an E-wallet
The unreliability of Curve acceptance meant that I always had to carry another card anyway.
The paid tiers offer little value for money. (for me at least)
Their only unique feature (as far as I can see) is “Go Back in Time”, which I’m sure before long, one of the more established E-wallets will adopt.
Like yourself, I see Curve struggling to attract new customers, or indeed retain existing ones in the very competitive FinTech market.