The way I normally do switches is I have my tank and off tank in the first two slots, who are slightly slower than the people in the third and fourth slot. (for boss battles it'll often be Meiling and 18f).
That means you can swap the last two slots every 25% delay gauge on your tanks.I dropped a dedicated 4th slot healer before plus disk, and started using only Reimu, so I just swap her out after exorcising barrier like any other nuker on my party.
I did this because I had a fragile nuke heavy team ( with characters like Kaguya, Flandre, Nitori, Suwako).
For Kaguya, I actually do use her buff once in awhile, for when I mess up a delay switch for some reason, or there's someone I really want to buff (like maybe my team is low on hp, kaguya is in 4th slot, reimu just used hakurei barrier. I buddha bowl Reimu, she exorcise barriers, then my two tanks get their turn and swap them out again).
But, I think if you use 16f, you don't need Kaguya's buff as much. If I didn't use her, I'd probably rely on it much more, and would have gone a mnd build. As it is, with a mag build, Kaguya's damage is pretty excellent.