For what you’re describing—holding Bitcoin, swapping to USDT occasionally, low fees, decentralized, and mobile—I’d recommend Best Wallet. It's a solid choice for long-term storage and provides a simple way to swap assets with low fees. It’s mobile-friendly, secure, and gives you decent control without too much complexity. Definitely worth checking out for your needs!
The best wallet I see non-custodial. Is Best Wallet Open Source?
The first thing I noticed in a wallet was open source. If it is not open source, I hesitate to store crypto in the long run.
If the best wallet supports a lot of crypto, I think the unstoppable wallet that I will recommend.
There is a discussion of discussion about Unstoppable Wallet to attend any discussion that helps increase knowledge about it.