I’ve also been having weird issues with the steam deck downloading for some reason and couldn’t figure it out. I use the dock so it’s using a wired connection (but the same thing happens on WiFi) and I have gigabit Internet, will get maybe 150mbps for a moment, then drops down to max 15mbps or slower but doesn’t outright stop like yours.

If you do a speed test in desktop mode via a browser what speeds do you get? I’m all but positive this is something specific to steams cdns. I also use a pihole but bypassed it for testing and it made no difference. What’s your download region set to in the steam settings? I’m on the east coast and tried a couple different nearby locations with no effect.