i know that the past character has the ability to "romance" callum and charlotte, but is there anyone else they could build a budding relationship with?
To be clear, there is currently no indication that the MC's relationship will Charlotte will ever be romantic, although it is possible for them to become closer to her. However, if that were ever to lead to romance, it would mean she was the secret RO, and we all know that's not the case, because the secret RO is Rowan Arcturus.
To make clear the distinction, the reason I haven't classified Callum as an RO despite his "romantic" content is that the prologue makes it clear that this relationship will not last and has no positive end state—at least not currently.
To answer the actual question posed, I can't really say with utmost certainty if there will be past opportunities for flings, fleeting courtships, or pseudo-romances besides Callum's, but my inclination is probably not, or at least none that are explored in particular detail.
This doesn't count Prince Willem, of course, who as we all know is the secret RO. The secret RO, as always, is exempt from all other rules, where romance is concerned.