It wasn't not enough power, it was 2 much power, Kim was afraid that the temporal device was going to explode. Not like a bomb perhaps but like a really bad day trying to drive to work and steam hooting out of your radiator... Or maybe a solar system levelling explosion, like Braxton did done?
Janeway brought tech with her that B'lanna couldn't install because it was "incompatible".
Arnour might have been cooler, but what Voyager really needed was a cloak and they've had that technology three times over, in Nightingale, Counterpoint, and Dark Frontier.
Remember that Miral tested the device which means she already went back in time, but whether that was a minute like in the Einstein strapped into the Delorian, or she had her own huge adventure in the past the writers assumed we couldn't be bothered with, but baby Paris had already seen history and got back to the future without any serious drama.
It had to be a one way trip because even if her time line still existed as a divergency, travelling forward in time would only return her to a new future history where another Admiral Janeway will be waiting to kill this woman who is obviously going to kill her older self because there can be only one.
It's selfish to alter the past for personal gain, but the Admiral was too stupid to see that she gained nothing fom this scenario because it is the other Janeway whose like is being enbiggened.