Well I am not an expert in this topic, but I think you should hire some blockchain developers in case you do not have enough knowledge in this subject, also blockchain developers are very well paid professionals, so surely it will not be cheap to hire them, but i think it is the best way to do this, I would do it in that way if I wanted to create an E-wallet for cryptocurrencies.
There is a difference between crypto wallet and an online wallet. For instance, paypal is basically an online wallet (it does make use of crypto but the feature is available on limited countries), however, Coinbase wallet is a crypto wallet. There is no difference between crypto wallet and crypto e-wallet. Some people prefer to use ewallet when they are referring to onlinbe wallet.
I was confused with what the OP was trying to say by the combination used in saying cryptocurrency e-wallets. I know e-wallets are different and I've used some of them. Google Pay is one of my favorites and I use it all the time. When I heard about cryptocurrency e-wallet, it sounded weird because I've not heard of it before.
