By “at the moment” you imply it will be, in the future. If so, and depending on budget, timeline etc (and whether you envisage re-locating Era100’s when you do want Atmos) I’d consider going to the 300’s now. I don’t know how much “better” the 100’s will be over your Ones. Or consider a pair of Fives, if best-quality stereo music is your current goal. 

Thanks...do you recommend pairing the set of ERA 300’s with either a sub mini or sub? Based on what I’ve read through my research, it seems like that might not be required and/or might be overkill for the space I have?

In most regions Sonos gives quite lengthy home trial periods. Buy and try the Eras. Get familiar with the sound. If there’s enough bass, all well and good. If not, add the Sub and trial it; keep or return it when your ears have made the decision. 

Not knowing your room size or acoustics, your musical taste, how near you are to neighbours, whether you like to boost bass or treble, how loud you like to play music, etc makes it tricky to give specific advice

In that large/open room the full Sub would likely do far more for you than the Mini.

300s or Fives is a tough call, maybe find a local dealer with a good listening room?

A pair of 300s beats a pair of 100s easily.   You can always add a Sub Mini later if that's what you want.   That would give you a better system that is Atmos capable if that ever becomes a thing. 

The important question, if Atmos is not a present need, is why upgrade. What is it that you think you are missing out on that the upgrade will give you? Unless you are clear about that, upgrading can be a bit like spending in pursuit of a mirage.

I have used the preceding model of the One, the play 1, for many years now for one zone among five, some of which are served by Connect Amps + HiFi level speakers. Once I added a Sub to the play 1 pair, it delivered sound of the same quality as said HiFi speakers. The requirement for the Sub to work well for music play is being able to locate it somewhere between the two play 1 units, so that the entire soundstage remains coherent.

Once your needs change to what this set up cannot give you, you can think of an upgrade around the Sub, replacing the One pair.