If I were your civics teacher grading you on an assignment, I'd ding your score for the year and the German flag. You could then successfully argue to me the year you chose was closer to mass uses of concentration camps, because that is more in line with your second sign. That strengthens my flag argument, but gets you half the points back that I'd have taken away.
Now, for the reality. This isn't an assignment; welcome to the both more and less forgiving real world. I see the first sign and know exactly what you're trying to say, and that's the real goal. It doesn't matter whether or not the year is mathematically perfectly analogous. Many Americans probably won't even recognize the modern German flag, so I would possibly recommend a Nazi flag (wow, words I never thought I'd say) in place of it for next time, but the fact that you're present is the most important part. I understand what your message is, so good job.
Being trans, I think for today's events, you'll be fine, but be aware and keep yourself safe.