I agree with replacing the bearings after this number of years, even if the trailer sat more than being used.
Since you are replacing the tires, I suggest fitting the tallest wheels that fit your current bolt pattern - You may find that buying a tire mounted on a rim is not much more than mounting new tires on the existing rims. That's what I found when replacing my enclosed trailer wheels. I was able to sell the old set locally and end up with new tires and rims for the same or less than just having new tires mounted on my original rims.
The kayaks may extend too far back off the trailer and be a safety issue without supplemental lighting. You can pick up a cheap set of magnetic mount lights and attach them to the kayak end with elastic cords. Run the lights in parallel with or instead of the trailer lights.