I found "Candy Alley" in Kawagoe to be underwhelming, although I'm not particularly interested in dagashi. I tend to agree that it's not your best bet. I really liked Kawagoe from the standpoint of architecture (period buildings), and I enjoyed the shrine there. But not the shopping, really.
I used to bring home enormous hauls of okashi (not the dagashi type, more like Kit-Kats, Pretz, candy, all sorts of rice crackers...), although I basically don't do it anymore. In my okashi-hauling days, though, I found the Aeon Laketown Mall in Saitama (Koshigaya Laketown) to be a treasure trove. For starters, there are two large supermarkets, each with a pretty good selection of okashi (including an aisle full of "modern" dagashi). Selection in each supermarket is slightly different and it's worth going to both although it is a serious hike to get to the farthest one. Then there are numerous other stores that carry interesting snacks and treats.
For dagashi specifically, there are a number of shops that have some, and two that are devoted to dagashi, the best one probably being icchoume ichibangai:
https://laketownkaze-aeonmall.com/shop/detail/152The other store devoted to dagashi is Dagashi Yumeya
https://www.aeon-laketown.jp/mori/shop/store/food-foo_347.htmlI am not personally familiar with either of these shops (or any other of the stores at Laketown that have dagashi), but you can probably get a pretty good haul just from those two.
Whether it's worth it to go all the way out to Laketown is debatable. It takes roughly an hour to get there from central Tokyo (depending on where you're coming from), and the tickets to get there cost money unless you have a JR pass. Furthermore, the place is so enormous that it takes a great deal of time to cover on foot, even if you know where you are going. If you just want to amble around and explore, count on at least half a day there. (And be aware that only a small percentage of the hundreds of stores there carry food items, let alone dagashi. But a few percent of 700 is still quite a number.) On the other hand, JR will take you right to the doorstep of the store. In comparison, it can take 30 minutes or more to get to say, Shinjuku Station from some other part of Tokyo, and then you have to walk all over the place to get to whatever part of Shinjuku you want to be in, sometimes in bad weather. So I find Laketown to be very efficient in a sense. The climate control is great, and so are the bathrooms, and other infrastructure. Lots of good places to get a meal or a snack to keep you fueled for the chase. And the supermarkets are definitely very good. Both are larger than nearly any supermarket in Tokyo. Anyway, I still go there on nearly every trip to Tokyo, to buy grocery items and a few other things. It saves me time and money, even factoring in the hit it makes on my Suica card (because the prices in the supermarkets are good). But I can only speak for myself.
If you decide to go out there, maybe bring a duffle bag or something to bring stuff back in, as supermarket "regi-bukuro" just aren't good enough to hold everything and can be tricky to manage on train coming back. There are free (coin-return) lockers at various places that can come in handy as you start to accumulate booty. (I usually buy a bunch of stuff at the supermarket closest to the entrance leave that in a nearby locker, and then hike out to the Aeon supermarket at the far other end of the mall, stopping here and there in between, and then consolidate everything before I head back to the train.) To save yourself some time, I suggest that you go to one of the information desks and ask the ladies there for the best stores for dagashi. Also, maybe pick up some cardboard boxes to pack your haul in. They have them at the Aeon supermarket, at least, and maybe at the other one. Use these as packing cases inside your suitcase and you can get a lot more in there, with no breakage. I always use cardboard boxes to bring back okashi. Wrapping them in clothing and such just doesn't cut it.
Finally, regarding Kit-Kats, they can be somewhat of a treasure hunt. Some of the regional ones are hard to find in Tokyo. There is a store in the Tokyo Station complex that carries a modest selection but I forgot the name (it has come up in numerous threads on this forum and also on TripAdvisor). It isn't bad but don't expect too much. To be honest, I've usually seen the best selection of Kit-Kats at the airport. It is a bit risky to wait until the very end to find specific ones, and you have to save room in your bags for them, plus still have some money left at the end of the trip. But I have never failed to see a few kinds at the airport that I didn't see anywhere on my travels around the country.
