I already elected COBRA following job loss. My 18-months of COBRA is scheduled to end in August. Can I get the premium subsidy until then?

Yes.  The COBRA premium subsidy is payable starting on April 1, 2021 until September 30, 2021 or until your COBRA eligibility ends whichever comes first.  So in your case, you will be able to get the subsidy for five months (April through August).