Furthermore I think he might even experience a slight bump in numbers because there'll be a number of people who don't particularly care one way or another about the Dreamers but who will approve of him for being decisive.
You are right to challenge my opinion given how much of it is instinct or a "gut feeling." However, I am in the U.S. and I see anecdotal evidence of Trump support slipping. (Family members, Republicans, have soured on him).
I also may be unduly influenced by the fact that I'm often literally surrounded by Mexican/American youngsters, some of them born in places like Alabama or Arkansas where their parents did hard labor, possibly alongside white workers who have found Mexicans aren't so scary once you know them. DACA is popular if polls are to be believed, so there's that.
Even if Trump's personal support is solid that does not necessarily translate into votes for congressional candidates who answer to constituents. Even if gerrymandering favors them, I hope they're becoming a little bit spooked by what the future holds if they continue with more-xenophobic-than-thou campaign platforms.
DACA people are embedded in the U.S. now, serving in our military, studying for health-care careers or just being normal American teenagers. Even though they can't vote, there will be support from many white people, some religious organizations , chambers of commerce, probably the U.S. military and of course other Latinos who do vote. IMO Trump himself doesn't really have a problem with letting them stay and work, go to school etc.
I'm not a wonk on how gerrymandering works but I gather that the "safe" districts actually aren't safe by huge margins. Tipping points can be reached. I'd like to know more about it - from your understanding or links you can recommend.