> Once it becomes self sustaining one should be able to add more fuel to the pellet to get more energy out for the same input energy.
That's not how ICF works. Plasma, being a gas-like state, will always expand to fill whatever volume is presented. With ignition, the rate of expansion is essentially lower than the rate of fusion, allowing you to fuse all of the fuel before the plasma dissipates and cools down.
In ICF as studied at NIF, you start with an extremely precisely machined piece of metal called a hohlraum, you put a solid pellet of fuel inside at an extremely precise location, then fire a laser with extremely precise alignment to heat the hohlraum until it generates X-Rays that heat the pellet just right so that its outer layer explodes, creating an equal implosion, generating two shockwaves inside the pellet; if the two shockwaves meet just right, at the center of their meeting place you get a fusion reaction, and you hope that that fusion reaction has enough time to heat up and cause more fusion reactions before the initial implosion loses speed and expansion happens.
That initial shock is the only thing containing the plasma - once it has lost its velocity, the plasma dissipates and cools down. If ignition was reached, the gas that cools down and dissipates should be 100% He, instead of a mix of He, D and T. However, there is no way to stop this dissipation, it is a fundamental part of ICF.
The only way to keep an ICF reactor going is to shoot one laser burst at one pellet, capture the energy of the fusion, and use that to power the next laser burst fired at the next pellet.
Of course, after each burst of laser heating the hohlraum so much that it radiates the heat as X rays, and then briefly containing a 1-10M kelvin burst of hot plasma, plus a neutron bombardment, the hohlraum is destroyed. Since machining the hohlraum to the precise shape required to achieve the shockwaves discussed above is never going to be a cheap process, it is impossible to imagine ICF would ever be even a tiny bit close to economical, even if it could in principle output more energy than it requires as input.
As such, ICF is strictly a scientific pursuit, mostly interesting for nuclear weapons research.