I haven't had the issue of casters feeling useless. Cantrips persist in being useful throughout and I am currently 3rd level, somewhere around the gnolls amd have rested probably twice, maybe three times. (I don't remember the conversation with Gale doing mirror image at the start happening on this run, but I got the convo with the devil just recently). I only ever rest when I start getting worried about a big fight coming. I can get past most fights without needing spell slots by judicious use of things like shove and cantrips. If I were to make any suggestion regarding rest it would be to make short-rests a two-per-long-rest thing instead of one, but even that's not really necessary.
