But most of my mains have short wavedashes. Kirby, Pichu, Pikachu, and Jiggs, so it's hard to do anything out of a wavedash with my mains. The longest wavedash of someone I main is Marth, but he's not my only character. I usually pick Pichu or Kirby and they both have short wavedashes, so it's super hard for me to make a good use out of their wavedashes.
If you claim to have ≥5 mains then you don't have any mains; you can play multiple characters but your main is the one you focus on the most by a large margin in practice and exclusively or almost-exclusively in tournament. Trying to "main" that many characters spreads yourself too thin and reduces both the speed and depth of your learning. You don't have to choose a main immediately, but you really should settle on one as soon as you can, or at least be in the process of doing so rather than trying to maintain that many characters at your peak and push them (especially if you're including Pichu and Kirby amongst them).
If you're talking about learning to apply wavedashing rather than just learning to do the input, then that's something that works differently for every character. Still, there are many things that wavedashing allows all characters to apply and benefit heavily from, including: grabbing the ledge by wavedashing to it, punishing a laggy attack on shield by wavedashing out of shield into something, and continuing a combo or tech chase by wavelanding onto a platform.
One universal thing to keep in mind (which is particularly important and widely applicable) is that dashing cuts off access to many of your standing options, and wavedashing allows you to quickly gain access to those options again by putting you in a standing state (and while still moving, unless you wavedash down or do a very short wavedash); while wavedashing is a movement option, it is also a way of changing the state that you're in, and thereby changing your available options. In general, understanding what each option you choose commits you to (both in terms of what it restricts and what it allows) is a very important fundamental skill, and something to consider constantly when evaluating your play.
Even the characters with the least application for wavedashing still have so much to apply it to, and they require it to be played at a high level; so you can certainly learn to apply wavedashing with any character. While you can learn and apply some specific things without understanding them ("I can wavedash down tilt to approach with Marth", "I can wavedash out of shield instead of rolling", etc.), properly implementing them (rather than just "doing them") requires understanding, and is a continuous process that will continue for as long as you keep playing or thinking about the game (and thereby further your understanding). If you focus on thinking about wavedashing on a more fundamental level (in terms of option coverage, commitment, advantage/disadvantage, etc.) rather than looking for specific things to do that you think the character should be doing, then you'll find it easier to implement fully and end up implementing it better in the long term.