Does your router have webinterface where you can monitor and potentially limit network usage of devices?
I'm not too familiar with the arr stack but if it is constantly downloading videos it's probably using full bandwidth. Maybe you can also limit in the arr settings or throttle the network interface on the laptop.
That's cool! I've always had the idea of a small k3s cluster on old phones with postmarketOS. I guess it doesn't work with older phones which don't have the latest Android Version but given the homelab trend generally goes towards small, low power devices, this could continue the trend with super small and low power phones. Probably in 2 years when current gen phones rotate out of company leasing contracts?