I'm not sure of the context for this, but if this is software that a given user will use a lot (as often is the case with enterprise invoicing) and therefore become an expert in, I would suggest focusing on what makes life easier and faster for an expert user rather than what looks cleaner or is easier for a new user. Split buttons and dropdowns are frustrating for expert users, and make the entire process much slower and prone to mistakes.
I would suggest using grouping, colour, and keyboard shortcuts to speed up the ease of use for expert users.
For grouping I would look at which (if any) buttons are logically most related to each other or logically used together. They don't have to be in a straight, neat line, and it may make more logical sense (you'd have to do some user research to find this out) to stack certain buttons to help indicate their logical relation to each other.
When there are several buttons, having different colours (or shades) can help expert users to quickly focus on the button that they need as they are looking for a green button for example. I did extensive testing years ago on a data rating system where there were 5 options, and we found that contrasting colours made a significant improvement to the speed and ease of rating for expert users.
Finally, expert users tend to use the keyboard a lot more than new users. Just watch an expert developer with their favourite editor and compare them to a new user of the same editor. I would strongly suggest adding clear keyboard shortcuts to each of the options so that expert users can quickly select them. If possible, I would suggest using mnemonic letters for the actions rather than numbers, as they are easier to remember and less prone to finger slip errors. So the key
Hold invoice rather than
Hold invoice. In terms of communicating that letter, there are several options to explore. For example, instead of "Hold invoice", you could use "Hold invoice" or "Hold invoice (H)". You'd have to include a tip somewhere that there are shortcuts, but expert users will love you for them.