If I were to build a deck based on Devotion, it'd be really helpful if I could search specifically for cards with lots of the same mana symbols in their mana cost. Is this possible? Alternatively, if I could arrange my search by "color weight", ie. if I search for 4 mana red cards, it'd put the (R)(R)(R)(R) cards first, and 3(R) cards last. Do you have any ideas how to do this?

On Deckbox, neither of those are possible, though the former has been oft requested. I'm not aware of any site that sorts by color "weight", not even Scryfall -- great idea, though!

The former is easily accomplished on Scryfall by searching for "mana>={R}{R}{R}". Note the >= syntax, so it'll find a card that costs {7}{R}{R}{R} as well as one that costs {1}{R}{R}{R}{R}. Also note the curly braces. For devotion specifically, you can add "is:permanent" to the search to further refine your results.

Not trying to draw traffic away from Deckbox, it's amazing at what it does, but when tool A doesn't do what you need it to, there's no shame in supplementing with tool B. :) Hopefully Deckbox gets some more advanced search features in the future to compete with Scryfall!