Start with the police union and go from there. I don't know how important it is to Ellison, but it may not be irrelevant.
He's probably not getting a lot calls from wealthy south Minneapolitans, but I'm pretty sure there are plenty of them that are annoyed by BLM and protest generally.
Some might be annoyed by BLM, but wealthy South Minneapolitans really aren't affected by the protests. Hell, normal Southsiders aren't either. If you were somehow able to ignore the news and social media and don't have to go to the Northside, you wouldn't even know this is going on. I live in Whittier and work downtown. The last few weeks haven't felt different at all, except for when there was the protest march to City Hall (I work kitty corner from there). A rich progressive living in Kenwood that's concerned about racial inequity would have it made - they get to have the issue raised without any of the mess on their streets.
Given that the 5th District is so incredibly blue, I don't think he has to appeal to moderates with his language. I also don't think he is worried about a challenge from the left. It would be hard for someone to look at his voting record (and being the co-chair of the Congressional Progressive Caucus) and really differentiate themselves as a viable candidate. Ellison was also part of the protests earlier, if I remember correctly. So it strikes me that, for better or for worse, he said what he said because he believed it. He doesn't have a lot to lose either way.