This was a little too boozy & bitter for some, so I was looking for a way to lengthen & sweeten it. So swapped out half the bourbon for limoncello & upped the orange juice. Found the result a refreshing bittersweet long drink for a summer's evening (recipe below)...

https://www.diffordsguide.com/cocktails/recipe/28758/holy-union

I agree. This is a near perfect Rum Old Fashioned...

Okay, after a few of these, I've decided to to not use any simple syrup as the drink is sweet enough already without it.

I'm using Copella cloudy apple juice and maybe it's already sweeter hence my decision. Great drink, but I think the extra sweetness is a muchness. All the same amounts otherwise

Agreed. Depending on the apple juice, I'd dial back the sugar syrup to 5ml or zero.

‘Man and boy I’ve built Old Fashioned cocktails these sixty years... and I have never yet had the perverted nastiness of mind to put fruit in an Old Fashioned.”—Lucius Beebe in the introduction of Crosby Gaige’s Cocktail Guide and Ladies’ Companion. 1945.

For your Mai Tai recipe, I'm swapping this out for Giffard's Orange Curucao (24%ABV)
Any idea how much this will change the flavour?