Note, I wrote this reply based on the title of this thread, without going through the entire post before I could detect the wings in taking such measures to store your crypto assets, because with the Opera cryptocurrency wallet, you don't own your keys, and without your keys, any coin store on that wallet is not yours but beings to the exchange who serves as custodial to those coins, so there is a lot to be done to achieve the financial freedom where you own your keys and your coins.
Avoid any form of centralisation It may seem to be the easy way but there are a lot of impending issues that come along with them.
From my understanding of what OP has said, Opera Mini wallet is a self custodial wallet but built on the web browser which OP is skeptical about. So OP wants to be sure if it would be safe to store assets on the wallet which there has been loots of opinion advising OP not to make such mistake doing it. Wallets were recommended for OP to give a try if it happens that OP wants to save funds and if OP should deem it fit, trying with little funds that one can afford to loss. This is best for OP to make decision.
