Umm… Europeans have been colonizing other Europeans long before 1492. The most notable example of this was Ireland:

Irishmen could not own land, sue in the king’s courts, hold office in central or local government, or be admitted to any ecclesiastical benefice in the territories under English control. In addition, the killing of an Irish man or woman was not a felony in English law; at most, the killer might owe compensation to the dead person’s lord.

This last provision did not, as is sometimes assumed, imply murderous intent. The point was that Irishmen, as aliens rather than subjects, were outside the protection of the law. But the implications of that principle, where settler and native shared the same territory, were far reaching.

(Source.)

Capitalist colonialism within Europe was phenomenal years before the Fascist era. A byspel of this was World War I:

In 1918 Germany annexed huge tracts of territory from the Russian Empire, taking direct control of almost all its coal mines, three‐quarters of its iron ore, half its industry, and a third of its rail system. An increasingly anti‐Slavic ideology added a racial dimension to this imperial expansion.

Generals Paul von Hindenburg and Erich Ludendorff wanted not only to control the resources of Eastern Europe, but also to subdue the region’s Slavic nationalities, settle Germans there, and create a “frontier wall of ‘physically and mentally healthy human beings.’” First in Poland then later further east, the German army commandeered forced labor, deported thousands of Slavic workers, and monitored the local population through registration and identity cards.⁴⁶

(Source and see Elusive Alliance: The German Occupation of Poland in World War I for more.)

If by ‘empire coming home’ you mean ‘white capitalists superexploiting their fellow white citizens’, then that is likewise a prefascist phenomenon:

Of the witnesses that Commissioner White examined (1863), 270 were under 18, 50 under 10, 10 only 8, and 5 only 6 years old. A range of the working‐day from 12 to 14 or 15 hours, night‐labour, irregular meal‐times, meals for the most part taken in the very workrooms that are pestilent with phosphorus. Dante would have found the worst horrors of his Inferno surpassed in this manufacture.

(Source.)

All that aside, what really disappoints me is seeing another person overlook the Fascist colonies in Afrasia. It bums me out. I try to regularly inform other users on that subject, so when I see a statement like ‘Fascism is the empire turning inward’, it makes me feel like my topics haven’t been of much help and haven’t really made a difference.

Fascism is a stage of colonial development, when the rate of imperial superprofit began to fall and the empire came home

But this didn't actually happen! Germany was crushed under the new WW1 order, wasn't fielding an imperial colonial army abroad, and it didn't come home. It emerged from the material conditions where it was, it did not leave and come back.

In fact, all the examples of the empire turning inward that we have are not examples that people would call fascism. For example, the system in the USA called "state police", which are different from local police, was a returning of the empire to their home in that the model for the state police was the design of the USA occupation forces in The Philippines. The rise of military weapons in the hands of USA cops is a direct returning of the empire to home, yet people are still talking about the USA as if it might become fascist later.

I understand the points you're making, I just don't think they reflect history at all.

and it’s not as if fascism and colonialism are truly different things; they’re part of the same ongoing process, two sides of the imperial boomerang.

I think they aren't different things - they are the same side of the process. I don't think there is an imperial boomerang. Again, I think the entire idea of the boomerang and the idea that fascism is when fascism becomes fascism is a white liberal ideological construction and doesn't match the material reality. If the only time its fascism is when powerful white people become oppressed, then that's not a useful analysis. White people are oppressed all the time in the USA and Europe - not to anywhere near the same degree, and not systemically/structurally on the basis of their racialized grouping, but it's undeniable that there are plenty of white people under the boot domestically.

When fascism “emerges” is when the revolutionary potential of the imperial core is at its highest

Again, also not borne out by history. You can say that fascism is deployed when there is a risk of revolution, but to say the potential is the highest is to ignore the reality that the states and periods we traditionally label as "fascist" did not exhibit any meaningful revolutionary potential.

As a way to define political moments its only useful as a way to understand revolutionary potential within the imperial core.

Maybe. This hasn't been shown though. The greatest revolutionary potential in the imperial core doesn't seem to be associated with anything like what happened in the Third Reich nor what happened to the American Indians nor what happened to Haiti. Instead, it seems to have been associated with labor organizing and with anti-war movements. Once European fascism materialized in the "Axis", revolutionary potential was gone.