How can I email a professor to ask if I am still being considered for an RA?
Given that you prefer an offer from them to all others, I suggest the following. 

First, the opening and closing remarks are a bit trite unless you know this person already. You aren't writing to a friend, so keep it professional. I sometimes use "Stay safe" as a closing salutation now days, but nothing else. 

But the important point is to leave out most of the detail. 


  I'm juggling some other offers now, but would very much prefer to accept one from you and would accept it if offered before I need to make a final decision. I wonder if I could have some decision from you by June 1. Also, thank you for the advice on things to do now. I'm following up on those and find them helpful. 


That is really all you need. The date you give should be the drop-dead date for the other decisions.