1) Load up Subaru with typical amount of luggage - empty, but you know what I mean. Bags and cases and such.
2) Schedule a bunch of test drives for an afternoon or a morning at a local CarMax. Queue up a bunch of recent model years for the vehicles you are interested in. You aren't going to be buying anything - these'll be used cars, and a model year or two older than current, but still good enough for this test.
3) All you're doing is seeing which ones will suitably accomodate the luggage (and analogs for the puppers & the puppers' things). To the parking lot where your Legacy is parked and back. Have you drive it over to the parking lot, get out and load the luggage, have your wife sit in the driver's seat and see if it'll adjust to her preferences. Both of these could be immediate deal breakers for a vehicle right off the bat.
You'll get some weird looks, but most of the time whoever you are dealing with is going to be game for it. And you should be able to schedule a bunch of specific vehicles in advance.
I did something similar when I was roadster shopping. Completely different segment, but similar concerns with luggage space - granted, my concern was "any" versus "lots"

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