The best way to choose a synagogue is to actually go there for shabbat.
Many wonderful shuls have lousy websites that do not reflect the warmth and atmosphere that are there IRL.
So give yourselves the next few weekends to try different congregations out. And don't worry, you're not married to your decision, you can continue to try out new ones through the year.
And yes, in many communities it can be very quiet in August, as many people go away, traveling, etc. Since R"H is right after labor day, that doesn't give you a great opportunity to choose where to be for high-holidays services, so its best to call or email the contacts and find out what children's programming they have, how welcoming they are to new families,etc.
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