Brilliant, thanks Simon.

Replacing the screwtop on a bitters bottle with a Dartdash does expose the bitters to oxygenation and will accelerate the evaporation of alcohol. If using a bottle in a few months then not an issue. Otherwise, perhaps replace the screwcap between uses. (If you do use a Dartdash then use a knife to prise out the plastic dasher insert in the bottle.)

You'd have to massively up the volume of the rum but it's not just the balance of alcohol to other ingredients, it's the flavour that overproof rum provides that makes the drink.