Sort of, but only if you exclude the amortised cost of fabrication from your calculation - it's part of the same goal, unlike say 'they're hard to make, we can't make them fast enough to put a dent in energy production'.

It is a total show-stopper because you don't have the amount of Tritium to power such a reactor.

We have plenty of lithium. All D-T fusion designs breed tritium from lithium, using the high-energy neutrons from the reaction.

I assume very little lithium is actually needed, will the dramatically increased demand from battery manufacturing affect feasibility?

Lithium production is a bit strained but lithium resources are fairly high. At Tesla's Battery Day they said just the lithium in Nevada is enough to electrify the entire US fleet. They also plan to do their own recycling, which they expect to work well since they can customize it to their battery.

Given the amount of energy that comes from fusion fuel, even extracting lithium from seawater would probably be economical.

Beryllium is much less common so that could be an issue. But maybe we can find more beryllium if we look harder; right now we barely use it. If we can't find enough, we could probably design a reactor to use lead instead. Either one acts as a neutron multiplier, and General Fusion's design uses lead instead of beryllium, for the same purpose.

DT fusion reactors require 6Li, which is 7.6% of natural lithium. The other 92.4% is 7Li. This "depleted lithium" would work in batteries just fine.