I don’t have any advice for your specific question, but sending heaps of empathy and I wanted to reassure you that although this issue of whether / how to stop feeding feels massive right now, that feeling will fade.
I had a sort of similar situation as I found out I had a brain tumour when I was seven months pregnant. I knew I’d need brain surgery - which would result in an extended stay in hospital - but didn’t know how long we’d be able to leave it. Baby was born and I breastfed her, but it turned out I’d have to have the surgery when she was five weeks.
I was devastated and spent so much time figuring out how I could try and keep my supply up while I was in hospital, how we could get her to take a bottle, how to stop her refusing the breast after weeks or months of bottle feeding etc etc. As it turned out, despite my best efforts, I was too ill to keep up my supply and when I came home when she was four months old she’d been on the bottle so long she was uninterested in going back to breastfeeding. Obviously this might not be your story, as your baby is older and you’ve established breastfeeding for much longer - but I wanted to reassure you that even if you can’t resume breastfeeding after your treatment, it will be ok. Although I still think breastfeeding is an amazing thing and wish I’d been able to feed my second daughter longer, I realise my health is more important.
Practically, in your situation, I think even though you don’t have a timescale for when you need to have stopped feeding, I’d start cutting down now. I think it’s better and less confusing for baby to avoid a sudden stop if possible. Also re not being able to hold baby - can you stay somewhere else during this time? I think it might be distressing for you both for them to be able to see you and you not hold them. I had to have surgery on my vocal cords a year or so after my daughter was born (it was a complication of my initial brain surgery) and had to be on total voice rest for a few days afterwards. I stayed at my mum’s as I thought it would be too confusing for her to see my there but me not being able to speak to her!
I wish you all the best for your health.