Definitely thinking out loud here, as I write this… But, maybe another way of thinking about this, is that a 5-string arpeggio is only “hard” to alternate pick if you can’t crosspick, but if you can it shouldn’t really be any harder than a 3-string one (assuming you’ve got the fingering down). So, one could argue (and I’m thinking through this as I type which is why I’m sticking to the passive voice, I’m not quite sure I’m ready to make this claim) that maybe the best way to practice techniques like crosspicking is to apply them to lines that can only be played in that manner, since at that point it becomes really easy to tell if you’re doing it right or doing it wrong. If you can’t play it, you’re doing it wrong and you need to change your approach. If you can, you’re doing it right, and should keep doing it.
Similar Devil’s Advocate sort of post - I switched from a mountain bike to a road bike ages ago, but I spent my teenage years on a mountain bike, and a lot of the guys I ride with these days also mountain bike when they’re not on their road bikes. One thing I remember clearly from my own riding and one thing they definitely tell me today is there are some things that are just not possible to do if you “start slow” and wait until it’s within reach. In particular, things like hitting drops are a great way to kill yourself if you roll up to the edge at a crawl and try to edge over - you’re going to land on your nose. Come in at a decent clip, though, ride up with authority, and send it, and “just whip it off,” and you’re probably going to land it, though. You need some idea of what you should be doing in the air, how you should be approaching, how you should try to land, etc… But you can’t practice that stuff at low speed, so you need to just focus on riding “fast and smooth,” and, well… whip it off.
I can certainly see some aspects of guitar being similar - if nothing else, the feeling of clearing a rock gargen on a mountain bike at a fast, flowing speed, vs. getting bogged down and jostled around and hung up on the rocks and roots at slow speed is an oddly apt analogy for alternate picking.