At 08.30 that should be the Superjet from Seajets. Normally this ferry is based in Ios for the summer. All depends if they will perform their normal route the day of the strike (3 September). If they do that, they probably will not be able to leave Piraeus again before 00.00 on Sept 4th. Question then is if they will go to Ios during the night to start their daily route.
BUT, in the past Seajets have surpassed that problem by taking Piraeus out of the route for 1 day. So, most probably there will be no problem getting from Santorini to Milos with that particular ferry.
Your other option that day is the Seajet2 at 13.10 which should not be a problem as that one starts in Piraeus in the morning after the strike is finished.
My guess is that both will run as normal. But it's still a guess. Don't shoot me if it turns out otherwise. One never knows with that union lead by communists.
Thank you Nicksy for the info! And I won't shoot you if I get stranded. LOL