Many things have diminishing return, so it's really relative.

I mean as bad as magatama is at lv 6, if you are at 100k power w/ that, you probably have such high level on other stuff like cave rune that magatama become a good investment again.

The trick is to keep things "even", don't spend all your resource in one place(unless it's a feature you just unlocked due to level up, as you'd be playing catch up then), so that everything would have roughly the same extend of diminishing return.

Also, the level of something affects the extend of how hard you should try to get it.

Take cave rune for example, if all of them are lv 5+, you shouldn't be spending coupons on them anymore, just farm keys from tree and get some from event. If the lowest is lv 4 thou, you should get the key pack mystery fruit, whenever they are offered. at lv 3 minimal, you should be buying key packs outright when they are sold, and finally, if you still have lv 2s, you should, at the next key rebate event, spend coupons to do 50 pulls until all of them are lv 3 or higher (you could stop when you are close if you have just passed a rebate point and the next rebate point is far far away, but that's getting complicated)

And you should probably have a tactics that's 1 lv below or even with your magatamas. Thou early on, you could consider not spending on it just to see which ones get lucky, so you can focus on those later.

It's just that clothing and breakthrough pills are special in that they don't have diminishing return, so they scale badly early compared to stuff that do (and haven't suffered much from it) but are very good later when most things have hit the wall hard.