It's not possible to just bill them whenever you want without tokenized billing, which we won't be implementing until later into CB 3.x rewrite of CBSubs. Even then triggering it irregularly risks triggering fraud detection and we'll likely heavily restrict this. I suggest just having recurring subscriptions for credits, which you can do now. I do not recommend trying to implement automatic "top-ups". There's good reason why it isn't being widely used across every app.

The best I could suggest is maybe automate generating a basket so on next login for example they can go right to manually paying. There's no way to trigger an automating payment like this though.

Do you have other ideas except trying to find another component ?

If one even exists then use what works best for you. I however am unaware of any payments extension for Joomla supporting this.

Easiest way to do this is to redirect their login to the addplanstobasket URL endpoint. This will automatically add the plan to buy credits to the basket and display the basket for them to pay. Example as follows.

Global
Triggers: onAfterUserLoginSuccess
Type: Redirect
User: Self
Access: All Non-Moderators
Conditions
Field: CREDITS_FIELD_HERE
Operator: Less Than
Value: 5
Action
URL:
Replace PLAN_ID_HERE with the ID of the plan you want them to buy.