You are confusing terms here. The question shouldn't be if Trust Wallet is decentralized. You should be asking if the wallet is open-source and/or non-custodial. Trust Wallet is non-custodial but it's not open-source. You have access to your recovery phrase and/or private keys, but no one knows if someone else has or what is happening in the codebase. That's the issue with any closed-source software or hardware wallet

I see no reason to hold bitcoin in a multi-currency wallet when there are so many good standalone clients available for various devices and operating systems. Even ETH can be safely stored on MetaMask, which is both non-custodial and open-source, and a decent choice on a clean system. Not to mention hardware wallets. No idea what kind of wallets are available for DOT or BNB though. A multi-coin client should be a last resort if you can't find better alternatives. Your first priority shouldn't be convenience and having only one wallet and one seed instead of multiple ones. It should be the safety of your coins.