Use the appropriate Colony Specialization to shift jobs between Alloy and Consumer Good production.
While this is clearly the right answer as the game currently is, the OP does highlight a weakness of the current setup of industrial districts: they are generally unwieldy and inefficent anywhere except a Forge World or a Factory World. This has several consequences:
1. The AI, and also players who haven't got the hang of specialized planets, can end up in a bit of a mess with regards to alloys vs CGs. The AI especially can spiral out of control with too many Artisans+Metallurgists (taking too many jobs and killing its mineral income) when it only actually needs more of one and not the other. (If the answer is "tell the AI to make specialized Forge/Factory Worlds", bear in mind that choosing the correct number and location of specialized colonies takes a lot more foresight, especially when most AIs won't have that many planets in the first place, and are always at risk of losing some to war.)
2. Managing the CG+alloy production of the capital is quite annoying (can be a factor especially early in the game and for certain origins). More generally, if you don't have many colonies, it's inconvenient that aside from shifting industrial policy, you can't make small adjustments to alloys versus CGs: each colony is basically all one or the other, or 50/50 inefficiently.
On the other hand, the advantages of having combined alloy/CG districts:
1. Less UI clutter. I don't think this is such a big issue at the moment, but it could incur a cost for the devs if they want to increase the total number of district types available later.
2. You can flip between alloy and CG production quite cheaply, by flipping specialization and maybe rebuilding the bonus building. This combines with the policy option of civilian/mixed/militarized economy, so it's only interesting if for some reason, changing policy isn't a suitable response to changes in demand.
With alloys, demand is very spiky, and there are certainly situations where you suddenly want a lot of alloys: war, or a new level of starbase becomes available, or you're about to build a megastructure. The situations where you'd suddenly need a lot more or fewer CGs, though, are a bit more dubious. Early on, colony ships, but then we run into the issue with lack of capital specialization. Researcher spam generally takes a while to build up to, and CGs are only a minor part of the cost (a larger part being simply the labour you have pulled out of other jobs to become Researchers). You could make a big change in living standards, but how often do you do this per game? Finally there's switching trade policies, which can change your demand for Artisans by quite a bit, but that also requires rebalancing your energy production, and flipping the Artisans to Metallurgists (or vice versa) isn't necessarily something you want to do at the same time.
Overall, there are pros and cons, but I think there's a reasonable case for splitting alloy and CG districts to simplify the game, since the two resources are quite dissimilar in what they are for, and especially it would help the AI out.