In terms of cost effectiveness.. if your cave runes are at least level 6, going all out on bells until at least your slot 1 is maxed is more cost effective than getting more cave keys/threads/refines with moon coins.

Unless you are spending a bunch of coupons on the bond scrolls, you'll get more moon coins than you are able to use for bells only. You'll have enough leftover moon coins for 1050 cave keys once per 2 or 3 months ish, depending on how much decisive bonds you do.

You are almost certainly better off getting medium refines/threads off monthly pts than to spend moon coins on them in the long run.