I generally agree with your statement that most of the targeted mass shootings have been perpetrated by young, heterosexual, white men. That said, I do have a few thoughts to offer.
First, regarding political affiliation and violence, the methodology used by most researchers, including the ADL data you cite, is flawed and skews the analysis. I’ve looked at their methods closely and much of the left-wing extremist violence is not included because it did not result in fatalities. For example, in 2020 the federal building in Portland, OR was attacked for 100 nights straight, and in total, over 200 days, by overtly left-wing extremists. They assaulted officers, used incendiary devices, broke windows, set fires, etc every night for over three straight months, but that isn’t counted by ADL as left-wing extremist violence. These attacks are categorized merely as public order disturbances, property crimes, or assaults, and thus not included in the ADL data, yet they most certainly represent extremist political violence.
In the last year, or so, we have seen left-wing extremist attacks become more lethal. I heard a great quote last night, paraphrasing here, the left uses violence like a dial, whereas the right uses it as a switch. Left wing extremists start with lower-level violence, riots, looting, property crimes, arson, and then increase the level of violence over time, like a dial. For those of you who remember the TEA Party days, you never saw riots, cars set on fire, vandalism, etc, but in contrast, right-wing extremist attacks, while far less frequent, tend to go directly to lethal attacks, flipping violence on like a switch. To me, however, it looks like the left has turned up that dial to where now they are at the level of lethal attacks, and the frequency is increasing.
The other point I want to make is that while historically, I agree, the mass shooter profile has been a young, heterosexual, white male, we are seeing an increase in transgender shooters. This is speculation on my part, but the use of testosterone for long periods of time for this purpose may be a contributing factor.
The last point I want to address is the differentiation between “mass shootings” as defined by advocacy groups and targeted mass shootings. The FBI defines a mass murder as one in which four or more are killed in a single event. This definition is then applied by many researchers and subsequently reported by violence prevention advocacy groups. As a result, many instances of what I will term street violence, that is to say violence associated with criminal activity (gangs, drug dealing, etc.) are swept up in their counts. One such shooting took place a couple of years ago in my hometown, Sacramento, when two rival gangs started shooting at each other late one night on a busy street as the bars and restaurants were closing. Tragic for sure, and yes, innocent bystanders were killed, but not the same as targeted mass murders where a specific location is attacked.