Proper repeat notation

There is no standard notation that means exactly that, of course. Best thing to do is write it all out - actually add 18 barst o the end of your chart, with a notation to fade wherever you want that to happen. That is clear and unamibuous and you won't waste rehearaal time explaining your intent. But simply adding a text message saying more or less exactly what you wrote abive has some chance of getting the job done, if you don't mind a bit of trial and error in rehearsal, and then repeating that experiencing each time a different group of musicians plays the piece. I find I have less and less for that sort of thing, and almost always write things out.