Keep, sell, donate — in that order, on purpose
Start with what's staying, not what's going — it's a smaller, faster decision and it removes the emotional weight from the rest of the sort. What's left splits into sell (real value, worth a quote) and donate (everything else, worth a receipt for tax purposes where applicable).
The house-cost side of the decision
A retirement-home-versus-staying-at-home cost comparison is worth doing honestly before the move, using the same carrying-cost logic as an empty-house calculation: tax, insurance, heat, hydro and upkeep on the current house, compared against the real monthly cost of the new living situation.
What a one-story house actually solves, and what it doesn't
Moving to a bungalow or a retirement setting solves the stairs problem. It doesn't automatically solve isolation, transportation, or the maintenance load of a still-large property — worth being honest about which problem the move is actually meant to fix.