How do I start my Sim off with max skills?

The only option you have is to cheat(google carls-sims 4 cheats) it or use a Sim from the gallery who have all skills and just change his appearance and traits (For traits I would only use the potion(Reward-Store 5000 points), traits change in CAS can sometime bugout and the game think the sim have still the removed traits)

If you have MC Command Center, you can go into MC Cheats -> Cheat Sim Info and there's an option in there to max all skills. Prepare to sit back and watch all the notifications for a minute when it happens!

(If not, or if you're on console, you can, as @daikoyu suggested, download a Sim from the gallery or cheat manually.)

Thanks!!!  🙂