I've gotten projection for distorted vocals like death metal and grindcore down pretty good, but I'm really struggling with projecting my cleaner vocals to sound like Sean Killian or John Connelly. I can barely get it louder than my regular talking voice. What practicing tips / techniques do you have to help with thrash metal vocal projection?

Fill your lungs I used to have this issue as well but it turned out I was just breathing wrong. If you fill your lungs properly and push from the diaphragm you shouldn't need that much effort to be loud. You can cheat a lot with mixerboard levels live but ime it will still sound weak in contrast to the instruments if you aren't maxing it. The hardest part for me with thrash vocals was nailing staccato measures at the proper volume without accidentally passing out, haha.

I also wouldn't recommend focusing too much on fixed ideals like Killian or Connelly before you know how your own voice sounds at full strength. In my teens I fucked my vocal cords over plenty of times trying to sound like my heroes, but at a certain volume you just can't hide your own tonal characteristics without tensioning the throat to "shape" the sound, which kinda works at lower volumes but will be damaging as soon as you add more force. I gotta say though, I've been doing cleanish thrash vocals (Araya-ish with a little more rasp) for ten years soon and sometimes I feel like I have no idea what I'm doing. But ever since I started breathing properly it doesn't hurt and I no longer get hoarse after rehersals/gigs, so it's better than what I was doing previously for sure.

Thanks.