Once more: It' possible to install Roon Server from Asus, why not MC?
It's simple, they don't support installing MC on NASes nor do they develop MC to do so. They were burned years ago when they had a QNAP version for awhile, which due to various issues got pulled and discontinued. So once bitten, twice shy. You can do a search of the forums for QNAP to read more about that one.
You might be able to use Docker, not sure with an Asustor, but that's a configuration not officially supported by JRiver so you'd be on your own there if anything happens.
IMO, it'd be easier to use a NUC and install Windows or Linux on that. Or even a small form factor PC with ITX motherboard in a small case. Or if it's audio only (which looks like is the case here) a Raspberry Pi would work fantastic. Honestly? I wouldn't buy a pre-built NAS chassis either (Synology, QNAP, Asustor, etc.), I'd instead opt to building my own NAS using TrueNAS or unRAID running on it, it'd be cheaper and likely more reliable. I'm also with zybex of the opinion that using SSDs would be a complete waste, as you'd get nowhere close to the advertised SSD speeds due to LAN speed limitations. That's where one of the benefits to building your own NAS would be allowing a Mellanox 40 gig network card to be used. Of course you'd have to upgrade your local network to take advantage of 40 gig too. But even then I'd find it to be a waste. *shrugs*
If you're just looking to playback audio, I find with my NAS, MC30 server on a Raspberry Pi 4 running Ubuntu 64-bit and my overall network (which both the NAS and Pi are connected via 1 gig direct ethernet to my 2 1 gig ports on my router) that playing back FLAC files (even 24-bit/192 kHz) on devices (especially devices that use Wi-Fi 6 and better) is almost instantaneous, using nothing but simple 7200 RPM hard drives. So yeah, for just audio it's even
harder to justify using SSDs because the majority of the performance is going to be wasted.