Welcome to the board! The admins would suggest that you consider introducing yourself in the "Hi I am..." section of the board.
I'm not a fantastic expert, but I believe that on your 2019 tax return, you will list your household size as whatever it is (you and your wife, and maybe any younger dependent children if you have any). Your ACA calculations will be based on what your household size is at the end of the year, meaning it will exclude your daughter.
Then you will list the subsidies you received further down on the form. Those subsidies will be compared to what you should have received based on your FPL (Federal Poverty Level) and the cost of the second lowest cost silver plan (SLCSP) in your zip code. Any excess subsidies will create a tax liability, and any shortfall of subsidies will create a refundable tax credit.
You may want to consider talking to the ACA folks about re-adjusting your APTC (advance premium tax credit, the monthly amount the government is sending to your insurance company) based on your new family size when she graduates. It sounds like you would get a lower APTC amount each month, but that would make the ACA part of your tax return closer to what it "should" be next spring and you'll be closer to the correct amount.