Prefer wifi network over iPhone

I've been struggling with download Audible audiobooks to my Apple Watch. They started downloading, but it was very slow and generally would not complete successfully. After a lot of investigation I finally found a solution, but it raises some questions.

I found that if I turn off my iPhone then the audiobook will download very quickly. This is because, if my iPhone is on, then the Apple Watch accesses the internet via its Bluetooth connection to my iPhone, and Bluetooth is quite slow. I verified this also by using a network speedtest app on my Apple Watch and monitoring network traffic on my wifi router. When my iPhone was turned on the Apple Watch showed no traffic going over its wifi connection. When I turned off my iPhone I could see the traffic.

BTW, when going though my iPhone I saw network speeds of around 2 Mbps, and when not through my iPhone it was around 30 Mbps. I do think the first number is quite low, though, since Bluetooth 5 is supposed to support 50 Mbps.

So, my main question is, is there any way to make the watch prefer to connect to wifi directly instead of going through the iPhone? And, related to this, why does it work the way it does? My guess is it is a power issue because Bluetooth uses less power that wifi, but that's just a guess.

Also, why is the connection through the iPhone so slow? Bluetooth 5 should support much faster that the 2 Mbps I'm seeing.

A follow up question is, is there anyway to make the watch connect directly to wifi without turning off my iPhone? I tried turning off Bluetooth, wifi, and cellular on my iPhone, but then the watch couldn't connect to the internet at all.