Congratulations on all the things! That’s a lot going on, and many great reasons to get a new MZ Wallace
You may find that once you need one MZW, additional bags are only logical...
Have you seen these bags in person yet?
I have never used Kate (overall design isn’t for me) but have observed a busy mother of multiple young children use it effectively. Personally, Belle is my favorite esthetic and experience (and feel during carry, when emptier it’s flatter) but in time I’ve admitted that most larger bags and non-backpacks are too large and heavy for my shoulders (they slope, nothing stays on).
That is all to say that both Kate and Belle are pretty large bags that can get heavy. Belle’s newer colors have the flat handle straps which help. The older rolled handles, still on Kate, will dig more, but Kate’s travel sleeve will help.
There are devotees here to Belle and to Kate. They say Kate is bigger both in profile and a bit in capacity. The bottle/elastic pockets certainly look convenient. Another consideration is whether you think more smaller pockets will be better for you versus the Belle’s larger flat pocket in front.
This is where I go back to the multiple bags comment. Different bags for different things. In any of our efforts to maximize efficiency (or is that justify more bags?), whether for weight or convenience or function, there is an MZW for everything. I mostly stopped using my own purse when I started carrying diapers and moved into a pouch wristlet wallet to go from work bag (usually backpack) to diaper bag (a medium MZ Wallace like Small Belle or an older style no longer in production ), even when traveling with or without baby, I just go super light weight and minimal cause laptop always weighs everything down.
Kate looks like an awesome travel bag. I just really love Belle’s design. Totally get your dilemma!