This issue of flushing the steam generators is a tough one to work out. The only company that really has this figured out is ThermaSol. Consider switching your generator to this model and limiting the amount of plumbing work needed to be done.
I would worry about splash out from the configuration you have there.
It would be better to have the copper to ABS connection above the floor grade and done with some kind of copper to abs fitting like a few reducers and MPT FPT fittings. I'm assuming the flush cycle is with cold water only.
What I like so much about the ThermaSol flushing is that it is automatic and flushes through the showers steam head. This draining and extra plumbing work for a flush cycle not needed.
Looking forward to seeing it come together.
What approach to vapour management are you taking?
Noble Seal TS is our favourite but you can go many routes.
Is that vent "Dry" or does the sink use this line some how?