I think along with English Cocker you've got a really good list here. I can chime in on LH whippets, which I think would check off a lot of your boxes! Although, I would love it if you got a koolie too, because we can never have enough koolie pictures
I prefer between 25-50 lbs, with 30-35 being ideal. Could go a bit bigger or smaller for the right dog, but nothing tiny or huge.
They're on the smaller end of your spectrum (they don't usually weigh more than 30 pounds, and some are as small as 20. Mine all weigh 27-30 lbs), but they're not little dogs - because of the sighthound shape they're taller than most other dogs of the same weight. Cricket jumps 16" in CPE, and Journey jumps 20".
Trainable, eager to please. I like intelligent but don't need it as long as the dog is relatively easy to teach. Rowan is problematically intelligent AND pretty easy to train; Riff is kind of dumb but also not very easy to train which I find to be a frustrating combo.
Definitely smart, and definitely easy to train. They do tend toward the softer side and don't work well with corrections, but as long as training is kept fun and positive they pick things up very fast. High food drive and many have high toy drive as well. Newton will work his heart out for a tug - I wish I had cultivated this more with Cricket and Journey.
I like a Velcro dog, but I have two dogs with SA already and am really tired of dealing with it. Ideally the dog would be happy to shadow me when I'm around, but be independent enough to handle being alone for periods of time (usually not more than 4 hours.
Velcro-ness tends to vary from dog to dog...generally they do not want to be up in your business 24/7, but definitely want to keep track of you. Cricket usually wants to be in the same room I'm in, but if she's comfortable somewhere she's not going to bother getting up to follow me around. The males do seem to be more velcro than the females, from what I've seen. Separation anxiety doesn't seem to be a common problem.
I really want an affectionate/cuddly person. I'm very touchy-feely and want a dog that can at least tolerate that, although I'd prefer they enjoy it too
Oh yes. They are very snuggly, love to be touched and cuddled with. If I'm sitting down I usually have a dog draped across me or curled up next to me.
I want a dog with enough drive and "oomph" to enjoy playing dog sports, but I don't want a dog that is so go-go-go that they need a job all the time. I'm willing to sacrifice some drive for that; I'd like to put a few titles on the dog (agility, maybe rally) but don't need a real high end competition dog.
They can make very successful performance dogs. They have more drive than a lot of sighthounds and have done well in many different sports (agility, flyball, rally, disc, freestyle, etc as well as things like lure coursing). Not as much "oomph" as some herding breeds will have, but all of mine are fast, have great focus, and a strong desire to work with people. Definitely a lot more drive than my rough collie had (though he was a rescue and probably not the most well-bred). All three of them compete (or are going to compete) in agility and I'd like to put some rally titles on them as well.
Also, really great off-switch. When they're on, they're on, but when we go inside they settle down very easily. They are athletic dogs and can handle lots of exercise, but don't require a ton for their mental well-being.
MUST GET ALONG WITH OTHER DOGS. This is huge. Rowan has a very low tolerance for rudeness, so while there's certainly a training aspect on this requirement (learning boundaries, proper greetings etc) I really need an easygoing dog that is ok with Rowan being a little bit of a bossy punk. This one is a little hard to pinpoint because his favorite dogs and some of his best friends are cattle dogs/cattle dog mixes which are just as pushy and snarky as him. However they've all had good dog sense; they seem to read each other well and never push over each other's thresholds. The real problem is crazy, excitable oblivious dogs (like sweet Riffers) who has no idea how obnoxious he's being.
I've never met a longhaired whippet with DA or really even DR. I know a couple different people who keep about ten in the house together with zero problems.
I prefer a coat I can bury my fingers in, so like a lab's fur or thicker/longer is my preference. I could go for a smooth coat for the right dog, it's just not my favorite. High grooming requirements are ok, not my favorite thing to deal with but it's a compromise I'd be willing to make.
Coat length varies a bit from dog to dog (mine all fall in the medium range, which is my preference) but their coats are very soft and silky. They do shed but not excessively, and grooming requirements are very low. They're pretty wash and wear.
Ideally will get along with cats in the house as well. I don't have one now but likely will in the dog's lifetime. They would never be left alone together, I just don't want the dog constantly chasing the cat either.
Ours get along very well with the cat; they do have high prey drive, but if they are raised with cats there shouldn't be any issues.
Temperament with strangers doesn't really matter. I like anything from aloof to extremely friendly. Rowan thinks everyone is his best friend
but I would be just as happy with a dog who didn't care.
Varies a little from dog to dog, but they are pretty outgoing in general. They'll happily greet and receive attention from strangers, but not to the extent of being everyone's best friend.
I'd LOVE a dog I could train to go hiking and stuff off leash.
They are more people-oriented than a lot of sighthounds, and although they have prey drive, if you work on a solid recall off-leash walks are entirely possible. Mine are almost always walked off-leash; they will hunt and chase after small animals if I let them, but they are also very reliable about calling off a chase and sticking close to me if I ask them to.
Now, unfortunately breeders are few and far between, and most are in the northeast. The breed community is very friendly and open, and because it's so small everyone knows everyone else. There's a big emphasis on performance - every breeder I know competes in at least lure coursing/racing as well as conformation, and over half do other sports as well (mostly agility and flyball). Not very many health problems, and breeders seem almost universally good about health testing - there should be genetic tests for MDR1 and CEA as well as a CERF and some kind of cardiac test (OFA or echocardiogram). Let me know if you have any questions!
Fun fact - I was very, very close to getting an English Cocker Spaniel puppy (had my name on a list and everything) before I discovered longhaired whippets. I still really like them and wouldn't mind owning one someday.