Could Australia's bushfires aerosols actually decrease global temperature?
Well, the amount and intensity of wild fires are a direct outcome of global warming, that's not really news.

According to this article that cites a work i can't find, until the mid of December 2019 the Australian fires emitted ~250million tons of carbon, ~50% of Australia's yearly production. A recovery in the coming decades is unlikely, because the ecosystem is already compromised. It is "too optimistic" to assume that the forests will regrow and they will "never end up accumulating as much carbon as they had before". Instead, large areas of burning forests must more and more be seen as a biospheric component that can turn from a carbon sink to a source.

Here's an example (sorry paywalled, but the abstract has the interesting points and it is peer reviewed) for boreal forest fires, where fires can lead from net accumulation over several fire events to a net loss, leading to a positive feedback. A similar process is expected for the Amazon rain forest fires.

To the question: an exact answer seems to be difficult. It depends on how much aerosols end up where in the atmosphere and stays for how long. The effect can be modelled: Here, a fire aerosol laiden atmosphere's lower 6km can be warmed because they become a heat trap. The trap is caused because aerosols have a stabilizing effect on the upper atmosphere. A forest fire's plume has a different composition than a volcanic eruption !

This paper describes a similar effect, the absorption of solar radiation by carbon can heat the atmosphere.

I am aware that there may be other effects, inclduing atmospheric cooling, but i leave that part to somebody else. All in all, the negative effects of carbon release and the potential loss of regenerative abilities of the biosphere - because of drought, erosion, loss of biodiversity and the ecosystem, soil sterilization, invasive organisms as well as the difficulties to respond in time because of the sheer area affected by fire (all depending on intensity, duration and area) - by far outweigh any cooling effects of wild fires.