I think coinbase is one of the safest places to store assets. because there are so many coinbase wallet users. and you should know that coinbase exchange ranks second after binance by coinmarketcap. so I think your assets will be safe there. and there's nothing to worry about that. And you don't have to look for another wallet.
Coinbase can able backup seed phrases but it is still an online wallet and their wallet is not open-source we do not know if our seed phrases are not recorded on their database so for me it won't be safe to use.
It would be better use a cold storage wallet or hardware wallet if you want a safest wallet.