GCSE year is most definitely the time to sort this out. I have one son in year 12 and one in year 9.
The rules were and still are, all homework must be at least started before any tech for fun. But I am a SAHM so I am here to enforce it but I don't have to do this anymore because it started out this way when they were in primary school.
Any whining or moaning about coming off tech results in no tech, any whining or moaning about leaving the house for whatever reason results in no tech.
Re GCSEs you need to sit them both down so your second child also gets their head around this and as PP suggested ask them what they want to achieve, where they want to go after year 11 is finished. How will they achieve this? How effective has their revision been? What have they done? Reading a revision book is useless unless you are testing yourself.
No phones in rooms overnight, and both my sons come off tech and read for a bit, this may well be a DnD mannual but it is reading still.
Totally agree with Beyond self care is essential, Ds1 is out of bed and straight into the shower at 6.30am. He doesn't struggle with this because he gets enough sleep for his body. Both my sons have stuff to do in the house, their rooms are spotless, laundry is in the laundry baskets, they strip their beds every week on a particular day and put it in the wash, they are responsible for emptying the bins, again on specific days. They help prepare dinner, they set the table, we all clear the table, no-one leaves the kitchen until everything is washed, wiped down etc.
We also spend time together after dinner, surely there is something you can watch together? We have just watched The Wrong Mans on BBC iplayer, Dh and I saw it when it came out. There are a million YouTube channels you must be able to find something you would all watch.
As you both work full time and your children are no doubt home earlier than you why are they not preparing a dinner? How hard is boiling some pasta? Or wrapping salmon in foil and putting it in the oven? You are raising children to be competent adults. They need to start taking on some of the household responsibilities. Ds1 was amazed at how easy dinner was in a slow cooker!
Big family meeting around the table. Decide beforehand how you want this to work.