Luxardo Maraschino has a very different flavor from Cherry Heering. I'd first reduce the amount of Heering, to keep it from overwhelming while preserving the original flavor of the cocktail. Swapping Luxardo for Heering would yield a different drink altogether.
I agree with Dave, I think the maraschino cherry syrup would be an okay substitute, particularly at just 1/4 oz. I think adding a teaspoon of Cynar or Averna, if you have one of those, will balance out the sweetness while adding the herbal component that the syrup is missing.
My go to for a regular Old Fashioned is Rittenhouse Rye.
For a bourbon Old Fashioned, I would go with a mid-tier bourbon such as Four Roses Small Batch, Elijah Craig Small Batch, New Riff Bottled in Bond, or for a more budget option Evan Williams Bottled in Bond or Old Grandad Bottled in Bond. In a cocktail where the spirit is so centered, the added flavor the mid-tier brings helps elevate the drink.
But you can also add spice and warmth by adding Angostura bitters instead of, or in addition to, the orange bitters. Bourbon + orange bitters is all about mellow, so turn to a rye whiskey (Rittenhouse) or high-rye bourbon (New Riff, Old Grandad) and Angostura bitters for warmth and spice.