The Sonos sub is really just a bass module not an deep rumbling loud sub, imo. I wouldn't worry about it and in most countries Sonos has a good return policy if you find it is too much. So look into that before buying, as a safety measure.
I wouldn't worry about the current sound bar being the only one that is compatible with Amazon music. I mean, you have a sound bar that works now with what you need. Maybe it dies in five years and what you want is a different music service altogether that doesn't even exist today. Etc.
That said, the Echo Studio is also expensive so I can understand: If you invest in those, you will be locked in for a while.
I would worry about the Echo Studio not being supported with new platforms in the future. I suspect Sonos will have a longer life from a support perspective and more choices for expanding in the future, and it is the CORE of what Sonos does and not just a pet project like Amazon's system.
Let me put it another way: Amazon could kill the Echo tomorrow and still be one of the largest most profitable companies on earth. Sonos could kill their product tomorrow, and go out of business. Who has more vested interest in keeping you happy, supporting your sound system, etc?
Personally, I like Sonos better in the sense that it is mostly a neutral player. Well, yes they have a little bias but their selling point is that they work with all the streaming services and I don't see that going away. I suspect Amazon is more trying to push you to be locked in to ONE music service?