I'm so sorry you and your family are going through this, and sincere condolences to all that have had to watch their loved one suffer. It's horrific. My DM had 3 brain mets, discovered when she realised she couldn't stand up. She tried and fell and ended up in a&e. She had a couple of procedures, one excising as much of the largest tumour as poss. The other was a laser focused radiotherapy. Neither worked, and she passed away 4 months after the mets had been diagnosed. What pisses me off the most is that she had a brain scan before she started treatment for her primary cancer and was told she had a couple of small lesions, but probably nothing to worry about.you asked in your OP what to expect, so I'll run through how it affected her below. If you don't want to read, I wish you all the best.
In the last few weeks before she died she was unable to walk unaided, lost the sight in her right eye, developed a slur. About a month after the laser treatment, she had a stroke, with all the usual symptoms. It left her unable to swallow solids, and there was slight personality change, she was extremely irritable, however that may have been the pain.
After the stroke we were told there was nothing more they could do, so was on palliative care. I managed to get her into a fantastic hospice who spoon fed, bathed, clothed and cared for her. In the week before she died she began to have fits that sapped the little energy she had. The decision was made to put her into a twilight sleep, where she wasn't in a coma but so heavily sedated that she may as well have been. I got to say goodbye and I love you and she passed away after 4 days.
If you've read all of that, I'm sorry you might have to witness this sort of thing too. Best of luck and take care x