I am referring to the Marxist-Leninist definition of the term, see these two texts: https://www.marxists.org/archive/lenin/works/1916/imp-hsc/ and https://www.marxists.org/archive/lenin/works/1915/s-w/index.htm
To be clear its not some cooky ideologically driven fantasy, the marxist analysis of imperialism and subsequent impacts on geopolitical and international political analysis is a well recognized analytical and theoretical model in IR theory.
From that perspective "imperialism" does not predate capitalism. It does not refer to "empires" in the vague sense of "Roman Empire, French empire, etc". The mechanics are vastly different. Aggressive expansion of feudal states in europe and their colonial expansion around the world (funnily enough that second one directly fueling feudalism's demise, serving as the "primitive accumulation of capital" that allowed the emergent bourgeois class to gain gradual economic hegemony, and eventual state hegemony) is not the same as the form of imperialism that emerges out of the most "advanced" expression of capitalism). It's understood as the monopoly stage of capital wherein bank and industrial capital merge, forming large scale monopolies, seeking new markets, and leading the state to engage in imperial plunder of less economically advanced states, and direct confrontation with other imperialist entities.
Furthermore, capitalism is positively not an "invention" nor is it dated to the 18th century ! Capitalism emerged organically from class struggle in the feudal period, with capitalistic elements emerging from within feudal society as early as the 15th century. It established itself as a dominant mode of production well into the 17th century in various areas of the world, but yes only fully superseded the feudal state structure and took control of the state as a whole in the 18th century. If anything was invented, it was the "word" for it, referring to what is an objectively observable scientific fact of human development (again, from the POV of marxist analysis, and its thesis of historical-materialism).
No worries at all, it's really my pleasure. As you said, establishing nonemclature within its theoretical and academic context really goes a long way, otherwise it's super easy to find oneself in a "dialogue amongst deafs`" (idk if it makes sense, it's a saying in my first language), and everyone speaks past each other while assuming the worst from one another.
Really happy to hear that other hexbear users were patient too, and I hope you get to brush up on your Marx notes (and include the Lenin texts I linked you, being serious they are very important, Marx only ever alluded to what Lenin ended up theorizing about !)