Chiming in to agree with both sides. I personally suggest you spend the time you'd be writing more of the same series on something new and different. You never know which book will land you an agent, and then you can share the others.
I actually did have another book already written in the 'vehicle dreams' series when I landed my agent. She liked it enough to mention it in a query to editors. But in the end, that particular book didn't get to be book two, Bulldozer took that spot, and then Fire Truck took book three's spot. The one I had written isn't in the series. I wouldn't call it time wasted, because I still hope that particular vehicle makes it into the series someday, but as you can see, even after a series is acquired things can change.
So, again long story short on agreeing with both sides.
If exploring other ideas in the series help you develop book one into its strongest potential, then explore those, as long as you don't use too much time or energy, and don't you get too involved with them, then by all means, explore them. But I wouldn't mention them in a query to an agent until you've made a connection. I didn't mention the second one I had written to my agent in the query. It was actually during our phone conversation that the other book came up. That was when SHE asked to see it.
But! I'd strongly advise using most of your time to explore new PBs. You can always circle back and write more if book one gets an offer.
