The issue with coolant was that the corrosion inhibitors would break down so cooling system metallic parts would oxidize (water + oxygen + heat = rust). Coolant and engine manufacturers are well on the conservative side so your 5 years (no matter what mileage) is too long but probably minimal damage and may never have a negative effect.
Most coolants such as Prestone are now good for 5 years so try to find one of those. If you decide to flush, use a garden hose and water but not much since you have to capture the runoff in a large container and dispose of it correctly (along with the drained coolant). I don't flush but change coolant every 2 years and now every 4 years. I use premixed since it is inexpensive, convenient and lasts at least 4-5 years. You cannot store it for the 5 years so buy just enough to do you vehicles. A 50/50 mix will work in most climates and in extreme climates you should never go beyond 40/60