The six-figure salary you gave up is an irrelevant red herring. Your story would be exactly the same if you had given up a cleaning job to look after your grandchild and then found yourself expected to continue even when her father had taken on a new partner and her children, and expected you to house them all too. The pair of them have three children to shelter, raise, and support now.
You can be pleased that your son has found happiness, but he has also taken on responsibilities - which do not over-ride those for his own daughter - and they are not your responsibilities..
Now that your son has a girl-friend in the house, he could create himself a family by integrating his daughter with the girl-friend's children. That would look to the future when you will no longer be able to do it! They could all live together (in harmony?) in the three bedrooms. Your grand-daughter could see you daily while waiting for her father to come home from work, but her step-mother-to-be could look after her other needs - as she would be doing if she was marrying a man without a mother to do it. If you wanted to, you could return to your previous employment.
Your half of the house should be as private as their part. It should not be taken for granted that one half can be entere3d without permission, while the other half is taboo.
I assume that the girlfriend is working. With two salaries, they could afford to give you more for rent - I take it you are doing this officially, with a rentbook and regular entries? If that feels unacceptable to them, then perhaps they would be happier being independent and owning or renting elsewhere. You have done your bit to help your son. If his new partner wants more than they can afford, she has an indulgent wealthy family of her own. It is their turn.
Meet up with them, and tell them these things - politely and tactfully, of course, but don't let them take advantage of you.