With the abundance of energy weapons, what powers all these ships and how does it sustain critical crew systems (gravity, oxygen, etc) atop those?

A combat titan usually has no crew aboard, a solid structure is much more resiliant than crew operated ships and allow for better manouverability.
The ship and its shards are controlled by the core ai and bound pilot compound conciousness.
Energy is gained through an exotic matter gateway dropped in the next stellar body, which creates a wormhole based plasma funnel (wormhole is stable for about 1.2 lightyears in distance to the gateway) the energy (most radiation types) is then absorbed and distributed by the ships kaydar crystal network, while the mass brought through the wormhole is funneled into the automatic factories, stored in the mass storages used for the engines or vented into space.
Energy is not so much of a problem, but heat distribution and the limiting factor of a titans combat ability (the reason why the kaydar crystals are relatively near the surface or have venting trenches.
Also many of the armor panels can be lifted or abandoned to provide more heat dissipation.

For deep space operations or when the gateway connection is damaged the ship has a set of fusion generators that are fueled by the ships mass storages. but this is just a tertiary system with limited ability.