The shortest tshog prayer I know for counting 100,000 is: Ganachakra putsa ho. For many years I was in a sangha that counted 100,000 tshog a couple of times per year. We used a 4-line prayer composed by Chatral Rinpoche specifically for our sangha for this purpose. Dzigar Kongtrul Rinpoche's sangha in Ward, CO also does this two times per year. They use a different short prayer (which I'm blanking at the moment). Neither my old sangha nor Dzigar Kongtrul's sangha offered/s a small plate with each repetition. So I would say that's a nice possibility but not an absolute necessity. I would recommend adding some fresh offerings at the beginning of each session during the day (if, for instance, you do 4-5 thun per day). Typically, one distributes the tshog offerings during the last session of the day and then starts the next day with a whole new array of fresh offerings. Also, the offerings should get more and more each day, not less and less, so that the last day is totally abbondanza.
This is a wonderful practice and it's wonderful that you would like to do it. I highly recommend it and hope that you do accomplish this.