When to invest into additional products?
Other than the generic calculation of "Present Value", there's no rule of thumb as such. Your decision should largely depend on your knowledge of the market, your capital and other production inputs, whether the licenses are substitutes, the marginal profit each one would bring, and so forth.

If a license has an expiration date, you might want to postpone the purchase till peek season is approaching. That way, you are not cash-strapping yourself unnecessarily. Also, there is always a possibility that some substitute and more affordable software might be released (or be noticed by you) sometime soon. For instance, would you buy a license if you find an open-source equivalent elsewhere?

On the other hand, the deployment of a specialized software might take time to stabilize, whence you would need to assess whether that would disrupt business operations.

Another aspect to consider is that by spending more of your remaining --and arguably vital-- resources, you are placing "more eggs in the same basket". That type of management is discouraged unless you have the certainty that your business will be profitable.

Knowingly or not, during your decision-making you'll inherently be applying many principles of economics and finance. There is just no tool or test that gives the precise type of guidance you are seeking.