About the iconography, our flag, etc. I agree with you, but if you use them it is very important to be clear and specific in whatever else the poster says, because vague statements can be very easily misread and those symbols by themselves will likely give comfort/anxiety to the wrong people. You can’t just wave an American flag and say “resist fascism” or “preserve women’s rights” or “protect children” or something like that, because those phrases mean different things to different people, and the color of the flag has been a useful way to know at a glance what the speaker intends.
From what I can see in conversations with Trump voters, it’s the specifics that give them pause – did I really vote for that? So, pick some particular action to respond to, or several in separate posters. There are so many to choose from.
I’m not sure what specifically is meant by phonics. My grandma taught first grade for 30 years, ending around 2000. She said when phonics came in “that’s just teaching reading” and when phonics went out “well, obviously we still have to teach how the alphabet works” and when phonics came in again “eye roll”. So, whatever the school leadership says, my guess is kids are learning phonics.