I wanted to focus on those sbc which are officially supported by OMV to avoid future problems
None of those boards is 'officially' supported and the most important stuff happens at lower layers (Armbian) anyway. It's just that we created OMV images in an automated fashion for some popular boards so users can save the hassles and time to install OMV in a second step (since this is rather time consuming with slow SD cards).
But even if there is no image for a specific board at the download page it's as easy as this: OMV4 on ARM boards (kind of a how-to)
Wrt new SBC: None of those I currently play with are cheap (all RK3399 or Marvell based)