Having a broad Ranker's Path was not that useful if Eren's mana core limited its potential.
It was like adding multiple functions to an Array Formation while trying to avoid increasing its operational cost or complexity. Or fitting multiple components into a flying vehicle and expecting it to deliver the same performance and fuel efficiency.
One couldn't just choose the best of the both worlds just because they wanted to.
The use of the pseudo-avatars not only helped Eren stay low-key within the confines of Sunhaven but also allowed him to look at all of his seven paths with a fresh perspective. It enabled him to approach the same problem that he had been having since his time in Anfang with a different mindset.
Eren's use of the "guinea pigs" in Siren's Sanctuary wasn't really one dimensional.