First, what is this "decision" you are making? And what do you mean by "add more"? And specifically WHICH relationship do you propose to add to?
Second, a point. Do your sex partners have other sex partners? If not, then YOU are nonmonogamous, but they (probably) aren't. Around here, we sometims refer to that as being monoamorous, satisfied remaining with one intimate partner.
I disagree that there are "many forms" of polyamory, a meme that leads to much confusion. There's various ways of mapping out relationships, which individually might or mightn't include sex or emotional connection of cohabitation or meeting IRL. For instance, a vee might live together, or the "arm" dyads may never meet over many years.
There's only 24 hours in a day, seven days in a week, 365.25 days in a year. Healthy people need 6-8 hours of uninterrupted sleep per night. You say you're a professional, which likely means 45-65 hours/week for work, out of 168.
What problems are you having with your girlfriends that you have enough time left to consider more relationships? How much depth with your extant sex partners are you willing to sacrifice -- maybe permanently -- in pursuit of novelty or maybe replacement? How have you explained to each of them that you intend to do this?