I think feel like the tree customs he listed (Grounding Blow, Guard Breaker, and Dash Counter) are unfortunately literally the only ones worth investing in.
Like, his UpB default is mandatory. You rest the KO and multi-hit threat and well as combo breaking that comes with his normal UpB, and while Rising Smash would retain threat but cut your recovery down to literally unacceptable levels.
Guard Breaker is honestly kind of dubious because on the ground it's so slow you pretty much have to catch someone committing to a Smash attack and armor-punish them, otherwise it's pointless. It technically makes your recovery better since now you have an armored sideB to return with, buuuut it's not a huge buff. I guess it'd be relevant against people like Villager. Dash Counter seems great... but I have no idea who you'd use it with.
The one legitimately, immediately tangible, benefitting custom is grounding blow, because you can punish SH aerials and even some full hop stuff which drastically improves Little Macs options for dealing with a wide range of characters in neutral. It's huge. But honestly if that's the best he gets, he's one of the least benefitted characters in Smash from customs, considering people like DK, Ganon, Bowser, Villager, and many others get an entirely new lease on life.