Thanks for summarizing this Seremond!
I think you’re right to say in your note that anything can be done after imbuing, simply at a greater cost.
This said, getting your LI at least to lvl 30 before imbuing is essential. This way, you have 6 legacies for free. After imbuing, you need to pay for each extra legacy.
Of my 2 LI, only one was fully prepared as described here. I chose to imbue both anyway. I could almost max out the one that was prepared, and it’s considerably more powerful now. The second is weaker than it was before, but it will gather iXP quickly while questing in the new area.