I think both approaches are too extreme. Supporting every device leads to poor security, poor stability, and therefore a poor user experience, but only supporting just Google devices (while there is a good reason for that) is a step too far for most people.

If I were in the position of e/os I’d just support probably three manufacturers. Going through the major ones that I know of: Motorola and Google are obvious picks. Next would need to be something cheap and popular. Samsung is way out of the question. Xiaomi and Vivo I’ve never seen their phones mentioned outside of China (which is a country that generally doesn’t have the same privacy considerations as people in the west do). That leaves Oneplus and Tecno Mobile for the third model.