By incident I got a very cheap used Ugreen DXP2800 without an actual need for a new NAS since my custom build NAS based on a J5040 CPU works just fine. But it was an offer I couldn't resist.
The installation of OMV couldn't be more straightforward:
1. I updated the preinstalled UGOS to the most recent version and did a factory reset (optional step)
2. Did a backup of the internal eMMC (32GB) using Clonezilla just in case I want to restore the UGOS partition at some point in the future (optional step)
3. Booted into BIOS (Ctrl + F2) and disabled the Watchdog
4. Installed OMV onto the eMMC (OMV can of course be installed to any other device and the boot order can also be changed in the BIOS)
5. Booted OMV 
Everything works perfect except for the HDD/LAN activity LEDs which can be fixed: https://github.com/miskcoo/ugreen_leds_controller It would be great to get the LED controller as a plugin at some point for OMV. Personally I don't care about the LEDs because I never look at my NAS once it is set up and running. The power LED works just fine out of the box.
The Ugreen will of course replace my custom build NAS because it does everything I need in a very slick and silent overall package. 