Sorry for what you all went through, death is always harder on the living,
Practically though there is zero chance assisted dying would have been used in your dads case. His was a relatively sudden onset and deterioration, if he had a DNR in place he would [hopefully] have had His wishes honoured. Anyone undergoing assisted dying is going to need all sorts of assessments and counselling.
However the one question I would ask you is … had you told your dad everything leading up to his cardiac arrest you’d wanted to say to him? or did that extra time give you and the rest of his family and loved ones time to say their last words? That he was able to hear and acknowledge. Hard as it is seeing him as you did don’t underestimate how important that process is.
Finally we are going to need to think about funding. Crass perhaps but the reality is the NHS has a finite pot of money to spend and we’re going to expect this to come from that NHS budget - that’s our healthcare system. There will be an army of professionals involved before anyone is even permitted to go ahead with this. Should the NHS prioritise assisted dying over some new quality of life preserving drug or treatment? Or should we ring fence the NHS as it is now? Fundamentally you’re changing its ethos if this came in to law and that should not be done lightly.