If you don’t like the Ugreen OS, you can install an alternative OS. I’ve tried OMV and apparently TerraMaster’s OS works just fine, assuming you can get hold of it somewhere.
You can find a guide on how to install Unraid here: Tutorial - Unraid on Ugreen
Unlike Synology and QNAP devices, the hardware isn’t locked down, so you can install whatever OS you want on an SSD.
From what I can tell on the instructions in UGOS, at the moment Rsync only works between Ugreen NAS devices, but it might be possible to bypass that via the command line?
You’re not wrong in that the platform is lacking in terms of features compared to Synology, but for how long have Synology been making NAS devices vs. Ugreen? Did you really expect feature parity? If so, I believe you bought the wrong product and should consider QNAP, as it’s the only NAS maker out there that will offer you some kind of feature parity. Asustor, TerraMaster and the others, will not be anywhere near what you’re used to.