...started the current write or carve on bullets thing.

...and related to crimes, not the military.

If they wanted to attack ICE, they would have shot the driver, immobilizing the van.

Acting on a plan, the shots were aimed at the interior of the van where detainees would be. Or it might have been empty.

Did the murderer observe detainees being loaded into the van? Probably. Or possibly unloading had begun. Any info about those possibilities?

...what I read earlier about him shooting indiscriminately at the building and sallyport.

Article about the shooting itself

https://www.cbsnews.com/news/ice-todd-lyons-dallas-shooting-indiscriminate-attack/

Article about what FBI has found

https://www.nbcnews.com/news/us-news/investigators-seize-dallas-ice-shooters-devices-rcna233708

and they were not the brightest bulb, took the instruction a bit too literally.

"Got it, boss!.... A-N-T-I-I-C-E."

Since the proud boys and other nazis have been getting employed as ICE agents, these weirdos have been filling the void.

Prior to Luigi, I'd never heard about it in real life, but remember it being used to id killers on several cop shows over the years.