Do you see some fundamental limit other than energy?

Containment, which is somewhat of a practical limit, and somewhat fundamental.

Now I'm no fusion expert, and frankly neither is any human since we can barely get tiny amounts of hydrogen to fuse at all. But point being, I'm not a physicist, I'm in software. So take my understanding with a grain of salt.

However, while we theoretically can fuse arbitrary elements (and add neutrons for heavier elements so they don't instantly decay), I'm pretty sure there's no known, theoretical, or speculated material or process that can contain and capture the results of fusing some elements to, say, uranium. The extraordinary energy levels required pretty much means you need a supernova. And I'm not one to suggest that in the future we'll magically be able to engineer something to contain supernovae. Quite the opposite, I'm comfortable saying no civilization will ever do this no matter how many quintillion years they try.

All this being said, there is definitely an inflection point where it costs orders of magnitude less energy to mine an element and rocket it to a destination than to fuse it on demand.

Sometimes people complain that this takes time and we need the thing now, but that's fine. Once you set up the supply line, all that really matters is throughout and interruptions, since arbitrary latency can be overcome by a continuous stream of material floating to you. I mean, sure, it'll cost latency to ramp up or down the stream, but if engineering is One Thing, it's trade offs. Reality's a bitch like that.

> I'm pretty sure there's no known, theoretical, or speculated material or process that can contain and capture the results of fusing some elements to, say, uranium.

Producing heavy elements in a lab is most certainly possible right now. You don’t need a supernova, just a particle accelerator. Many elements heavier than uranium have been created artificially this way.

https://en.wikipedia.org/wiki/Transuranium_element

You're right. But synthesizing enough material to be relevant to an industrial effort is a leap of engineering so large as to be beyond our capacity to imagine its possibility.

With the way that resource allocation seems to be bound by presently, if there was a war, and one/some group could some how get the resources to scale up such processes if they could seriously convince the war planners of its "super weapon" capabilities, I could see it happening even if it took a couple years to get a working prototype at scale (depending on its ability to not compete sufficiently with other resources for war to not purse such project at all). And seeing that humans haven't came up with the abilities to avoid war completely, it's only a matter of time (which may be a lot for an individual humans lifespan).