To be honest I sometimes think gardens are a bit of a red herring in dog ownership - they seem to result in walk-avoidance over time.
I agree with this. Secure gardens are good for toileting without needing to leash/control the dog, but in terms of entertainment or exercise a garden is next to useless unless you use it for specific activities. e.g. set up an agility course in yours. Unless you have a field, the dog will quickly get used to the space so it provides little stimulation.
OP, as with all cross breeds. Look at the very worst of both breeds and make sure you can handle/live with it. Because your dog is as likely to have the worst of both breeds as it is to have the best.
Collies: incredible athletes prone to obsessive or compulsive behaviours, such as obsessing over balls, bike-chasing, herding or nipping children etc. Need to be well socialised because they are not naturally friendly to all.
Labs: bouncy, sheds a lot, greedy so will steal food (out of children's hands) without training, energetic, plenty of health problems in the breed (e.g. hips) so this needs checking, chew all the wrong things as puppies.