For every level-up that you attained in hobby, you would get a level-up bonus as below:





Therefore, once your Sim reached max level and getting 1 LP, make the Sim switch to another hobby to forget and then switch back to build up the level again. This way, you will start earning the level-up bonus again.
E.g.: I've max my Sim in woodworking, I made him do ghost hunting and upon completion, he has become a ghost hunter. Now I switch him back to woodworking and build up his woodworking skills from level 1 again.
NOTE: This forget-&-restart hobby should only be done on ghost hunting, figure skating and woodworking only. As these 3 hobbies are faster to level-up (with ghost hunting the fastest)