I would say that Imperialism overlaps Fascism, but (one of the) difference(s) between the two is that Fascism goes a lot harder into Ultranationalism. Under Imperialism you have a King or Queen, and they are ordained by God to rule. Under Ultranationalist Fascism your race (or whatever they count as ‘race’, nation, etc) is what makes you superior to everyone else. It was a lot harder to get people to fight for Kings and Queens they only knew from coins, than to say “we’re fighting those EVIL (other nation people), whereas we are the PURE nation.”

Another difference is that Fascism adores and requires total war. Imperialism wants to expand, but Fascism wants to dedicate every aspect of the nation towards that goal. There’s that enigmatic ‘other’ that has to be destroyed, because it is both a weak and strong opponent. Fascism also says ‘violence is good for the nation, if directed properly.’ This means your January 6’s, your political assassinations, etc, are all highlighted as good things. ‘Drain the swamp’ kind of rhetoric becomes literal violence to allow people being killed.

As mentioned below, Franco’s Spain was quite bizarre, in that it had a lot of different traits (including fascist ones).

Imperialist nations can abandon overseas colonies, ‘let’ them become independent, etc. Yet Fascist nations need invasion and war to keep their economy going, which means they have to pick bigger fights. They also relied on slave labour (and I’m going to take this opportunity to name and shame the corporations that used those people as slave labour:

Among the slave laborers in the occupied territories, hundreds of thousands were used by leading German corporations including Thyssen, Krupp, IG Farben, Bosch, Blaupunkt, Daimler-Benz, Demag, Henschel, Junkers, Messerschmitt, Siemens, and Volkswagen, as well as the Dutch corporation Philips.

What people forget about Germany is that they had always planned to invade the Soviet Union. There’s a lot of talk about ‘if they hadn’t,’ but their lebensraum plan required it. In 1944 75% of their economy went to the military. They’d been deficit spending every year since the early/mid 30’s. Fascism requires that all effort be put towards the military and war, regardless of if you’re at war, but you need to be at war to get the land/money/etc you need to pay for the military that you made to… etc.

So anyway yes, fascist nations bully countries until either a bunch of them or one big one puts them back down.