Definitely go for a rescue if you can find a suitable dog. Please go to a rescue that vets very carefully, as some rescues, whilst well meaning, often get it wrong and with a young family you don't want to have to be returning an unsuitable match. Many rescues do have fosterers that allow the dogs to be seen in a home environment as you are never going to see a dog's true personality whilst in a kennel environment.
Its very difficult to give a breed suggestion as dogs are little individuals but there are traits to be aware of.
Collies and Shepherds can tend towards nervousness and need a lot of stimulation, not just exercise and don't sound like they would fit in with your lifestyle.
Labradors are lovely dogs, need exercise but will also fit in with what you describe. Do be careful about choosing the right breeder, do you want a working line or a show line? a working line will obviously need more exercise. They don't really qualify as medium size but are quiet in the house so wouldn't be such a large presence.
Spaniels do great in a family environment and are very popular but they absolutely will struggle with only two short walks a day. They can be really very strong and need early lead training so you are not walking a steam train.
AVOID any brachycephalic breed like the plague (anything with a deformed face, like a pug, bull dog etc). Very popular at the moment and subsequently suffering from over breeding.
I like cocker poos - generally I have found them to be friendly and easy to "do" and they do seem to fit into family life well. Again, because they are not an actual breed you might fall foul of "backyard breeders".
I think id be drawn to Labradors in your position. There will be posters who will agree and those who don't. Its so very personal.
Good luck with your research and hopefully doggie acquisition soon.