Alright @jec0995 now we're talking!
First, let's talk about option 2, which is upgrading your Long Haul Trucker. If you are on a size 54 or larger, I believe you can swap over to 700c wheels. This will help you in your goals of riding centuries and having an efficient commuter. Upgrading wheels and parts will take all of your $600 or more to be honest. The LHT is a nice bike and if you love it, then I could help talk you through upgrades. If you are on a size where you can't swap in 700c wheels, I would look for a newer bike based on what you want to do.
Now if we are looking at a new bike, here are my thoughts.
- Co-op ADV 2.2 is on sale currently and fits your price range. It has rear rack mounts or a spacious frame to add a frame bag for commuting. The larger tire clearance and 40mm tires will make your commute a dream and allow you to venture off-road with your wife and son should the pavement end. You can always swap to 32mm tires and have a nimble century machine as well! The Shimano GRX group has been getting a lot of great feedback from folks since it came out last year and should really go the distance.
- Cannondale Synapse - I linked to a specific model here because it is about the only one that will fit in your price range. We have limited sizing available, but if it lines up, you could snag a good deal! The big difference between the Synapse and ADV is going to be tire clearance and end use. The Synapse is an endurance road bike at heart that can hold a slightly wider tire. The ADV is true mixed terrain bike than can be do road duty with slimmer rubber should you want it. You still have rear rack mounts available to you as well.
- Trek Checkpoint - One thing I will point out is the geometry on this model. It will be much more stretched out than the other bikes you are looking at. Just keep this in mind. For a similar size, you will have to reach further and be in a lower or more aggressive position comparatively speaking.
Let's keep the conversation going based on how this info sounds to you!