Although it wasn't as difficult to make as the Philosopher's Stone, to the extent that one wouldn't even dare to attempt it—at least these two potions were still within Salvatore's knowledge.
But at the time, Annan was preparing to challenge the Spirit Monk, and Salvatore was very worried that if something went wrong, he might drag Annan down.
Therefore, even if it meant breaking his own self-imposed rules and risking his body being taken away, he still decided to release the shadow.
After all, the shadow was also a facet of Salvatore. Even though it was a personality opposite to Salvatore's, he trusted her not to cause trouble at a critical moment.
And indeed, the shadow did not cause any trouble.
But Salvatore felt that this technique of "suddenly shaking a substitute into existence" was quite useful, so he became somewhat addicted to it.