I think the path Serenity took is the better one. Initially targeting QEMU as the single supported platform. You have the same advantage as far as targeting a single platform for drivers but contributors don't need to buy additional hardware, can develop using the platform/tools they are accustomed to, starting instances is faster than rebooting hardware and no need to deal with the issues of remotely debugging. Targeting a specific SBC as a 2nd platform after a certain level of stability is reached is probably a good idea.