When I read your OP I did think that a herding dog might be a good fit. I'm a herding breed nerd, so it's not uncommon to see 'your breed' when people describe what it is they're looking for. I have an Aussie as a sport/performance dog (out of pet/show lines). She's a firecracker and loads of fun. My last Aussie was my family dog and he was so laid back and relaxed. Aussies can be awesome, but more and more I'm seeing a lack of play drive/handler focus, and some members of the breed have quite the stubborn streak. Honestly, it again boils down to the individuals and the lines within the breed. Getting to know the sire/dam, the lines the breeder breeds from, etc will go a long way to helping you find exactly what you're looking for.
Or you can go to a shelter and find a cool ball-obsessed lab mix and have a great companion.
Add me to the herder-brain club. A few of your points did strike me for a herder. I will caution towards the exercise needs as well, however. Just be very honest with your breeder. You will have to watch prey drive with these guys though... just saying. IME, they will be attracted to all of the small animals. I know several live with small animals and are fine, but you get a squirrel running and you are going to have to have a decent recall.
Also, jobs are important to these dogs. So if you are just looking for a companion without giving them something specific to do it may not be the right fit. I know that is super helpful.
May be worth looking into a general or breed specific reputable herding breed rescue. I know it's a ways out, but HeRD of Wyoming is a great resource and they may have some leads for you.
Also be aware that ESs should have a good bit of hunting instinct, so if you'd be disturbed by those small mammals maybe being dinner once in awhile, just be warned lol
Just emphasizing the prey drive again.
I have a Boxer as well (and have met PLENTY more). They weren't the first thing that popped into my mind while reading your description, but I don't think they are a
bad fit persay.
They fit your bit of "judging strangers" pretty well. Mine is a pretty good judge of character. With that said, the socialization is super important. You want to make sure they meet several people and yadda yadda, otherwise you may end up with that instinct going the wrong way. They are definitely watchful of their home and she will alert to things that are weird long before my herder will.
They are loyal to their person. Even though I put all the training on my girl, she's my husband's dog through and through. She would do anything for him. She's kind of middle ground on velco-level (though you have to remember I live with ACD's
my judgement may be off). She likes to be around, but doesn't HAVE to be. They are dogs that like to be doing things with you. She loves plowing snow in the winter with Josh. One of her favorite activities. Not super prone to SA, but it can happen.
She's not a wanderer at all. She likes to stick close to the house and to us when we go hiking. However, biddability isn't there. She will work for super awesome treats, but to just work because she wants to? Nah. Most Boxers I meet don't have the best recall, either... mine included. She's offleash if we are in a rural area because she stays close, but other than that she's on a lead. She's got no interest in small animals and no prey drive. Toy drive is something that is touch and go. I think most prefer tugging to fetch. I think your exercise limits are pretty reasonable for a Boxer if you go with the right lines. Health-wise, I'd definitely go with health tested lines. Dog reactivity is something to watch for in the breed, as well.
I know this is long winded. I'm sorry! Just trying to help. I think with the right moderate conformation, health tested line Boxer you would find a nice dog that fits most of your needs.