You mention that your SO enjoys coming to the park with you "most days" so why not just go on the days that they are willing to go? Don't think of it as depriving the dogs - they're still going to get way more free-running playtime than most dogs, and are in a far better position than they were with past owners.
On the days you don't have access to a vehicle, take them on local walks on leash, or see it there is a closer park where dog owners congregate where they can socialize if they are not too shy or aggressive.