@Cora
No-one, even the healthiest, knows how long they have. Most of us choose to extend the time we have, if we can, as far as we consider reasonable. It’s a bit like age - the notion of what is reasonable gets more elastic with time just as the notion of what is old gets stretched out too. My mother used to refer to the ‘oldies’ at her aged care home, that categorically did not include her (88) only those in their nineties. We don’t get to choose the time we live in either, only what we can do in it.
Your two choices seem to assume that a) all treatment will be unpleasant and b) that you will be able to live your life to the full without treatment - ie no further unpleasant illness or incapacitation. Neither may be true. There are other options. Give treatment a try (all or some) and see how you fare with it. You may be surprised. You might get more information from your oncologist or surgeon about options and projections.
You may also want to consider your views about the really hard question - facing death. None of us get out of here alive, as the saying goes but it often takes something like cancer to start thinking about our mortality seriously.
Your decision is yours, not any one else’s not even your partner. You are the one who has to be content with it, no regrets. Best wishes.