Well, you could get your test data from paid participants who consent to constant whole-home recording. Like the "Nielsen Families" of old. This would be worse

Well, you could get your test data from paid participants who consent to constant whole-home recording. Like the "Nielsen Families" of old. This would be worse data than just using all available inputs to train your model, but it's an alternative. There are alternatives to storing all input data for later training; they likely produce worse models and cost more, but they are available.