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If crucial is a bit too "strong" for OP's purposes, I'd have thought the same would apply to vital, necessary, essential, etc., in that they all imply the process simply couldn't take place at all without the charge.

I therefore suggest it's important, which admits of the possibility that the process could take place without the charge, but by implication it would be less efficient, slower, or something. Personally I would also accept central as a "watered down" version of crucial, but I think some people would say that's another "vital" word.

Overcomplicate and convolute are fine but they have slight negative connotations. If you're looking for more neutral words, from Oxford dictionary, and don't want to use to complexify (which is exactly the word that fits your definition) for some reason :

To sophisticate

(transitive) to make more complex or refined

or also

To refine

To make or become free from coarse characteristics;

Make or become elegant or polished (intransitive; often followed by on or upon)

to improve (upon) by making subtle or fine distinctions

Other synonyms at these entries could work depending on the context, such as to fine-tune (a concept), to complexify, to improve, to perfect, to polish, to modernize, to upgrade...