For starting a tribe, we would need a certain amount of villagers to be reproductively viable, as well as a large stockpile of resources and food. To join one, you would need to travel to an island with a tribe on it and both make friends with the tribe and make a substantial contribution to their stockpile, with say food, steel tools, and firearms.
I personally recommend repairing an abandoned boat on a random island rather than building one from scratch. Of course to get to that island, you would need a sailraft but that wouldnt be enough to get you back to the mainland. Perhaps have a chance of success based on the provisions and repairs you make before setting sail?
For the signal fire, it should take many logs to build and only last a short while. It would be possible to build on the fifth day if you focus exclusively on it and get lucky with materials. Each day after that, with a larger chance at the end of the month and only once per month, there should be a chance that a plane would fly overhead at sometime during the day, and you would only have a short time to light the signal fire before it leaves.
One key aspect in some of these plans is traveling between islands, but that could be easily accomplished and provide an interesting challenge. You could travel to a new random island with a sailraft and start with the provisions you put inside of it. That would give you a head start on the new island, which is important since some may be lacking a particular resource such as coal or various plants. For each travel, you would have a chance to land on one of the tribal or boat islands and get a chance to win the game. This seems to me like an excellent way to improve the endgame.