Serbs invaded Croatia after they used their constitutional right to leave Yugoslavia and retain former borders.
(ASIDE: It is ironic that since independence in 1991 Croats are very anti-communist but always refer to borders drawn by… communists! And you do, too: “retain former borders”)
Yes, while Croats have worked towards secession from Yugoslavia in 1990/1991, Serbs on parts of territory of that federal unit took guns. I am not denying it. But why? Are they some wild or crazy people, willing to kill and be killed? Or were they afraid that Croats were preparing to continue doing what they couldn’t finish in 1945, for example in the Concentration camp Jasenovac [1]? Look at the photographs of killed people in that WIkipedia entry. See that knife that Croats have used to kill the Serbs faster (you've called it “Srbosjek”/”Serbcutter” and have made it for faster throat-slitting).
Were Serbs afraid for no reason? Were they just imagining a Croatian boogeyman? After all, Franjo Tuđman, the Croatian President at that time, was a communist and partisan. It turns out, there is an audio from a meeting of the Croatian leadership prior to Operation Storm [2]. In that audio Tuđman can be heard saying “da nanesemo takve udarce da Srbi praktično nestanu” or in English “to inflict such blows that the Serbs practically disappear” [3]. I could name (if you want) many other examples in the 1990-1995 timeframe that show the Serbs were afraid for a reason.
You personally are an example for what I write about the whole time, as well. You have admitted your family were actually Christian-Orthodox Serbs (“My family used to be Orthodox and now my side is Catholic.”) and now you are a Roman-Catholic Croat that defends everything Croatian. Even here and now we can observe Croatian ethno-genesis before our own eyes, which lasts for hundreds of years: How Serbs turn into Croats and why that causes problems, both to those that take the new Croatian identity but also for those who remain Serbs. Those who remain Serbs are witnesses which, by their sheer existence (sometimes words) remind those that have turned their backs to their forebears to become something else that is, by design, anti-Serbian.
It is thus not surprising that all key people in the first Croatian state (1941-1945; see my other comment) were all Serbs a few generations earlier (Ante Pavelić, Ljubo Miloš, Andrija Artuković, Mile Budak and countless and nameless others). Yet to become something else they have had to kill those that have remained Serbs using the most brutal methods - by organizing throat-slitting competitions or by crushing their victims' heads with a wooden sledgehammer, to name a few. They had to burn the identity-bridge behind them, they had to kill the Serb in themselves.
[0] https://en.m.wikipedia.org/wiki/1991_Croatian_census
[1] https://en.wikipedia.org/wiki/Jasenovac_concentration_camp
As I remember the early 90s, the Federal Yugoslavian Army -- which, with all the other republics seceding had in practice become synonymous with Serbia -- rolled in with tanks and artillery.
So I think "invaded" was meant literally, not as a synonym for immigration by ethnic Serbs.