Beto does not support Medicare for All and has a
history of rhetoric talking about 'cutting wasteful spending' and being open to weakening Social Security. He's not left-wing on these issues and even if he claims to be now, there's no history to corroborate that.
And him making immigration his #1 issue is certainly fine - but if you care about the long-term safety and security of present and future immigrants to this country, I'd advise you support candidates willing to explicitly detail how they'll go HAM to combat climate change, because a future where climate change ravages the world is a future where the right-wing position gradually just openly becomes "exterminate anyone crossing our borders to protect our resources." Electing a standard incrementalist liberal is just giving us the equivalent of 10 more minutes on the clock. Better, sure, but people need to demand a candidate that has more systemic, far-reaching solutions more than just wanting someone who looks like he could play the part of the President on the West Wing or something.