Celfable has a max CP of 2353 and Crobat has a max CP of 2466.
You could max each of them and it wouldn’t cause them to de-motivate too quickly, and would increase their Machamp-walling abilities.
I’m not sure about their bulkpoints but I’d power them up whenever you’re itching to power something up, say at least to 30 and then make a project out of it.
Do you have other high tier gym defenders aside from the “Big 4”? Gardevoir, Slowbro/Slowking, Muk, etc.