A lot of those are south of Mykonos, so not quite on the route
you initially asked about.
However, if you are happy to go south for a bit, you can anchor
off of Delos (nice and quiet after the day tripper boats are gone).
Also Despotika at the end of Andi Paros is nice and secluded(ish).
BTW. Naousa isn't strictly inaccessible by car, and the water
sports boats cruising around the anchorage during the day could be counter to what you are looking for, but it is quite lovely from dusk onwards when all the holiday makers head
back to their hotels and holiday apartments (there isn't a taverna at Monestari Beach, but the bar cafe does stay open until about 6).
Can thoroughly recommend Livada beach on Schinoussa also.
You may find it a struggle to find secluded beaches anywhere in Greece
during the summer, but even with a few other boats in the anchorages they are usually all very peaceful after dusk.