I paid for sub but still no renewal

[Niwashi]

Is buying Crystas worth the spent?

It doesn't make any difference to the cost. The number of crysta you need to pay for something costs the same as paying for that thing directly. So, for example, a one month standard subscription that costs $14.99 would instead cost 1499 crysta. But 1499 crysta also costs $14.99. Each crysta is one cent, both in terms of what it costs, and in terms of what it can pay for.

You just need to check how much it's going to cost before you buy the crysta (remembering to include things like extra retainer fees if you use them), so you know how many crysta to buy. And it adds an extra step to the process, since you first buy crysta, and then pay for the subscription using those crysta.