Wheow! Thats a lot of music files! You planning on starting a streaming service?
Well, your Synology is best suited as a storage unit since it uses an Atom processor. And id seriously consider a custom built Roon Server with increased networking bandwith than a single Ethernet port can provide. As for a processor i’d never even look at anything else than a Kaby Lake i7, a minimum of 16Gb RAM and perhaps an Optane memory solution?
Concerning your second question, it’s not a problem, at least to me. Indexing of further albums/tracks is quick and painless. but on the other hand you have about 20times more tracks than me!