Ideas for new (to me ) family bike?

I've a Pinnacle Dolomite I got for commuting on roads/light "gravel" off road, which was fine on 28mm slicks year round. Now I have my daughter in a hamax seat on the rear rack so I fitted 30mm G one speeds for more confidence off road. It's fine whem not very muddy, but her friends seated on mountain bikes definitely don't get rattled around as much! A gravel bike frame would allow fatter tyres & still be quick on road, but finding frames with a load limit over 20kg was a struggle (without going heavier than I want to). & I'm not too sure how long she'll be in the seat before we start pushing the load limit. Like you, a trailer is in the future as she recently got twin brothers. So I'm thinking a trailer on my mtb & a hybrid/mtb for the wife that can carry a 3 year old in a seat. 🤷‍♂️
Back to your question, I've definitely found a drop bar bike quicker than a flat bar hybrid when commuting - a ribble cgr or kinesis tripster are potentials if I wanted a new 'all rounder'

As soon as your kids are able to pedal, get one of these - https://roland-werk.com/addbike/, then they can do their shift as a stoker. Brilliant things, I used ours when my son was age 7-10 and he was powerful enough to push both of us along at a decent speed on the flat. The rack it's sold with is very sturdy and will take a child seat too.

There's also the FollowMe tandem (I think reviewed here) as an option - great for taking the kids further from home as they can unhitch and ride independently, and then hitch up when they inevitably get tired. 

I would view this as an opportunity to acquire two bikes! 

I found as the kids got heavier, that the leverage of a wide flat bar made it much easier to keep the bike upright when stationary.

So, I'd look at getting a nice new endurance/gravel type road bike for the commute, plus a second hand 26" wheel MTB for kid and shopping lugging duties. The smaller MTB wheels keep the kids seat weight lower for a better centre of gravity, plus you can fit wider tyres for a more comfortable ride.

You will probably spend a lot more time on the commuter bike, so I'd try and avoid compromising it's performance by trying to get it to do everything.

I think you need to think carefully about what you mean "turn of speed", but also be realistic about what you're going to carry and for how long.

Pretty much anything with drop bars will probably have a "turn of speed" on a Sirrus, but a 14kg (unladen) tourer like the Galaxy is going to be.... marginal... in that regard.  Which is why I mentioned how much load capacity you are going to need.  At ~20kg, the kids are approaching the max load capacity for most rack mounted seats, another year and they'll be above it.  So, at that point will you really need the ~50kg cargo capacity of the Galaxy?  The new arrival alone won't require it.

If you're going to actually use it as a car replacement, loaded up with shopping and stuff, even without the kids, a second hand Galaxy is an awesomely practical bike, but it's slow and heavy compared to most other drop bar bikes.  In this category, also consider a Spa Cycles 725 tourer or pretty much anything by Surly.

If you don't need that capacity long term, then a value spec gravel bike like the Boardman ADV 8.9, Ribble CGR AL or (for a sportier option) Boardman CXR 8.9 will be much more fun when lightly or un-loaded, whilst still having eyelets for mudguards and racks

What is your budget?

Have a look at the Boardman range, they do some very good bikes, their cyclo cross bike may hit the sweet spot for you.