Bose Security and Apple AirPlay2

Bose just announced they're going to maintain limited functionality on their SoundTouch speaker line but they're ending security updates. To function as a multi-room Apple AirPlay2 speaker it needs to be on the same wifi network as our phones. Ending security updates concerns me. I'm planning to create a parental control group on my wifi router for my SoundTouches and give them no internet access. I tried it last night and Apple AirPlay2 still works with phone on my network but the Bose SoundTouches can't access the internet. Do people have other thoughts for keeping their home networks secure?