If you have a perfectly usable NAS and want to run a Roon setup then instead of spending money on a new NAS I would get a cheap NUC (or similar) and use that instead for Roon. The performance will be significantly better too.
Your assumption is right: the DS416 familiy uses an ARM CPU, the exception being the 416Play which uses a 64-bit Intel Celeron CPU.
While Roonserver will run on that model, please be informed that it is below the recommended system specs and will give sub-par performance on any moderately sized library (say: 15.000 tracks and over).
You would be much better served by an Intel i3 or i5 based server in any form or shape, with an SSD for storing Roon’s database.