In your thinking, separate rare coins (numismatics?) and collectables from Investments. Sets and rarities carry a premium that you may not get back in the future. Rarity/beauty value may go either way- a gain or a loss.
For my children, several years ago I bought a number of older sovereigns for each of them, with no collectable value. They are worth simply the gold value. But although that may or not keep place with inflation, they are easily sold and carry no taxes.
Several companies specialise in this, and offer multiples of 5,10, 25 etc.
In all cases there will be a buy/sell spread, but life was ever thus.