Once you're happy, grab Sims4Studio and merge the files you want to keep. This is how I whittled 18GB down to just over 1GB. You'll be surprised what you thought you needed that you never actually use, and you still have it safe 'just in case'.
Because your game will load so much quicker now, as soon as you start playing and think, 'Oh hey I need this hair, or that top,' you can grab it from your old Mods folder and it won't take long to get back in game. Pop your must-have gameplay mods in your new empty Mods folder, like MCCC, NPCC, default skins etc. Next, move all your CC to a new folder, out of TS4 directory. Starting from scratch is always best, and you can save any families you particularly like in your library. That way, any townies, other Sims etc wearing CC won't be affected. I recommend you create a new Save game first of all.
