My dad had a glioblastoma in his temporal lobe (right, i think) when he was 68, died 11 months later aged 69.
It was about 4.5cm when diagnosed. Unique to my dad, he had had a head CT, for investigation of something totally unrelated, 12 months before his diagnosis, and there was NOTHING on that scan. So in under a year that tumour had started growing and reached 4.5cm in diameter.
He had surgery, the doctor wouldn't confirm a diagnosis (or even make a sensible/ obvious evaluation) until the surgery with biopsy, when from the previous scan info it was clear it was as aggressive as they come. The surgery left him paralysed down one side. In the two or three weeks prior to the surgery he was on steroids and he just went back to normal (in hindsight he'd had weird symptoms for months), and we did some nice things together as a family. After surgery he was disabled, depressed and his presenting symptoms worse. I wish he'd never had surgery and just had longer being 'him' with less time overall.
He was in hospital about 10 weeks after surgery, then after discharged started radiotherapy with low dose chemo tablets alongside. He breezed through this with just a bit of tiredness, totally unphased.
After radiotherapy he went on to the higher dose 4 weekly chemo (tablets at home), he was so sick! Week 1 he was taking chemo and was nauseous and a bit sick, week 2 he couldn't get out of bed with the nauseous, vomiting and fatigue, week 3 he was just exhausted and run down from the previous 2 weeks, week 4 he was okay. My DM and i talked behind his back, that this was insane and no life for someone who was destined to die relatively soon anyway. I spoke to my dad, he said he was worried that if he didn't continue the chemo that people would judge him for giving up and letting them down if he stopped the chemo, i said the only people's opinions who mattered were his, DM, me and my DB and that me and DM felt he should stop, and I'd talk to my DB but was sure he'd agree. Dad didn't have any more chemo. (This was in February i think).
In late May he started having seizures, in late June he went into hospice for respite care as neither of them were coping at home, he was brewing a chest infection at the time so was started on oral amoxicillin. That didn't work. Throughout his illness i had talk to dad about his right to refuse treatment including antibiotics (DM, DB and me were all LPoA for health but dad never lost capacity). The antibiotics didn't work so he was offered stronger alternative antibiotic, and admission to hospital for IV antibiotics. He declined and passed away 2 or 3 days later at the hospice. He was still only taking paracetamol for pain relief at this time.
My advice is, that this is a rapidly fatal condition whatever treatment is thrown at it, so don't let your parents be pressured into any treatment that your dad doesn't actually want just for the sake of protocol, culture and quantity of life over quality. Focus on booking some cool activities as a family rather than putting all the energy into treatment that will ultimately be futule We went to Longleat, he did a disability friendly holiday with DB and family, we got him a disability friendly flight in a 2 seater plane for his birthday, i got married and found a hotel which was fully adapted and provided carers for the weekend so he could attend.
Sorry you are going through this too x