So frustrating. This is a huge job just to get down to the leak, then to not take the time to clean off the old gasket material after all that work. What a waste of effort.
I would go back to the dealer and ask them to re-do the work. This is an extremely expensive repair and the old gasket material should have been removed before installation of the new material, and they clearly did not do that. So, they are at fault for this leak. If they give you any grief, call Toyota. Toyota paid them to do this repair, and Toyota will make them re-work the job since they did not do it right.
Going forward, I would not repair a cam tower leak until it got really bad. It is something like a $3-4000 repair and not something I would attempt on my own.
I would certainly not be concerned about it until I had oil actually dripping on my garage floor, and even then, I would put down some cardboard or one of those metal oil sheets you can get at an auto parts store. The oil leak is usually so small that if you pay attention to your oil level, it should never get to a point where you have a big enough leak to make the oil level go low. There is no pressure in this area of the engine, and the oil is just return flowing back to the pan, so it is not like the oil is going to get pumped out or gush out. And when it came time to trade it in, get some gunk, clean that area well and hose it off and move on with your life (because if they see oil in that area when you trade your truck in, they are going to ding you the cost of the repair against your trade in value).
If the cam tower leak got bad, I think the first thing I would try is to cut away the excess sealant that is visible on the outside of the tower and clean that area with a strong degreaser like simple greeen, and then take a bead of JB Weld and spread that across the area that is leaking and go as far as is practical along that seal, then see what happens. The JB Weld might crack or break over time, and it might fail, but it also might work well. If that failed, I would then remove the JB weld and use a more flexible type of seal in that area. You might try just covering that area with a good quality gasket maker material and see what happens. There is no pressure there, so all you are sealing is a path way for the oil to leak out so it does not require much of a seal to work.
Good luck with it.