Something to remember about CGing is when you need to JC grab them when they DI behind you have to hit backwards VERY fast when you go to begin the dash behind or you lag in turnaround for a little bit and will miss the grab usually.
It's particularly important for Bowser's since he's so heavy you have barely enough time to turn for 1 frame, dash for 1, jump for 1, then begin the grab when he's at very low damage. The extreme slowness of the throw for him also requires getting used to if you aren't to make sure you don't try to begin the grab before you can start to move.
If you take longer than 1 frame to go from neutral to like 3/4 of the way behind on the analog you instead take 6-7 frames to turnaround before you actually begin dashing behind. When this happens you usually just end up grabbing in front of you since you JC your turnaround into the grab instead of JCing the dash that doesn't come out.
It's probably good to make it a habit to really mash the analog behind you when doing it, especially if your analog is more sensitive and/or stiffer than most controllers.
For spacies at really low damage I'd recommend d-throw to jab and JC regrab or some other follow up, or d-throw then run and get right ontop of them and forward-b in either direction as they hit the ground for an easy techchase. If they hit the ground with a bit of space between you a d-smash just as they touch the ground will also hit them if they do anything but techroll away, while an ontop forward-b away or down-b away will cover all but a techroll behind. If a platform is right in front/above you you can u-throw them to be able to punish their landing on the platform if they DI away or get a hit if they DI elsewhere.
At mid damage you can do the same techchasing with d-throw, d-throw to f-tilt if near the edge and they DI towards it, u-throw to a platform, or try to CG with d-throw if Falco (don't bother trying it on Fox really). If you grab them at like 100%+ a d-throw will easily set up an f-tilt regardless of DI, or if you react quickly a down-b will get away DI or no DI.
U-throw isn't very good to use when there isn't a platform they'd land on if they DI away and have room to tech away since they hit the ground further away from you and you can't cover their landing options as well as you would with a d-throw which keeps them close where you can get 3 out of the 4 in one move instead of 1 or 2 and be able to react to a techroll behind.
