The ST will work like the server it is and the NAS will be nothing but storage for the Roon software running on the ST. No applications running on the NAS at all.
As far as I understood you, the NAS and the RoonServer, aka ST, are plugged via ethernet into the router, and the BlueSounds connect to the router via Wifi. Yes that will work. Your environment and the music resolution and format types might have an impact on the quality of the Wifi connections.
I bought my sti5 today… will soon have a setup like yours. I guess you don’t yet have Roon on the Bluesound, so are you streaming to or via the Bluesound app ATM?