Solo ENM, might be moving from concept to reality

I'm a 40s single mom, and I parent half my time. In my non-parenting time I was interested in having some variety of relationship again. Since I'm not interested in introducing anyone to kiddos anytime soon, I decided ENM and FWB was probably the way to go.

I got lucky to find a lovely FWB and it's been going great for months. He is also solo ENM, and has seen other people in the time we've known each other, which is fine with me. I did a lot of thinking and what I really want is to feel like we both value and enjoy our time together and make time to be together - that there's something special and we're celebrating it. I am clear that I can see other people, but so far I haven't gone looking. I felt satisfied and busy enough.

And then I just so happened to meet someone who seems great. And now my mind is swirling with trying to figure out how I even figure out if I have the time and energy to explore a new connection and how I could do it without anyone (me, my work, kiddos, FWB, new interest) being hurt or feeling like they're not getting enough of my time, energy, and attention. The obvious answers are honesty with myself and lots of communication.

I don't even fully know what I'm asking. I guess my questions are: How do you help yourself figure out what saturated looks like for you? How do you keep existing good relationships on their excellent path when NRE happens? How do I help myself with the transition from theory (this is what I want and I can care about and enjoy multiple people) to reality (I am actually successfully caring about and enjoying multiple people)?

It's probably all somewhat complicated by the fact that FWB and I do have a D/s component, which is my first experience there and is wonderful! I don't want to mess with what's good, and I have a hard time imagining being submissive with anyone else, though I'm sure that day will come.

TL; DR: thinking about actually having a second partner and wondering how not to hurt anyone, myself included!