Doors – Tips for removing drywall from bottom of closet header

So about two years ago, I added a header to make my closets standard height, instead of being full 8' doors (which, even without doors, made the rooms look much more modern). Unfortunately, I made a rookie mistake on the first one I did, forgetting that drywall would add 1/2", and so the opening on one door is 80" (instead of standard 80.5").

I bought some sliding doors which I was hoping would fit: they have a small amount of room to adjust up/down, but unfortunately, it's not enough.

What I need to do is remove the drywall so I can mount the tracks directly to the 2×4 underneath, giving me the extra 1/2" clearance. What I didn't think of is that the metal corner bead would be in the way. I've taken out the drywall and scraped the paint/compound off the area I need to cut, so now I have this:

I tried cutting into the bead with a Dremel multi-max, but it's slow-going, and not really feasible for the whole thing (not to mention, I'll probably go through a ton of blades). I have another rotary tool (like a normal Dremel) with a cutting blade, but before I try it I figured I'd ask here to see if there are any better ideas (and I'm a bit worried about sparks from it).

I want to avoid damaging the actual corner, and certainly avoid doing any drywall finishing. There is a piece of trim that will cover the front cut (trim is 3/8" thick), and the back will be hidden behind the track itself.

So any suggestions?

Also, obviously I'll have to remove the screws. There MAY be a couple more under the part that's staying, I can't remember. Will this be a problem in the future without screws on the bottom edge?

Here's what the doors look like finished, in another room:

So to rule out a couple suggestions:

• Custom doors (even just 1/2" shorter) were double the price, not worthwhile
• I don't think I can get away with cutting them down. The edge is already pretty narrow, and even if I can get 3/8 or 1/2" off the bottom, it will probably look lopsided. Taking any from the top will require redoing all the sliding mount points, which I'm not sure is even possible, and is certainly more work than what I'm doing now

Follow up (after answered)

I ended up doing it with a cutting blade on a regular rotary tool.

Took a while, went through 1.5 blades, but got the job done.

I also had to build a small mount for the trim. The tracks for these doors have double-sided tape on one side, and the trim just sticks to it. Since the track in my case was recessed 1/2" under the drywall, there wasn't enough area left for the trim to mount to. I took some scrap metal I had, and cut several pieces that stick up 1/2" beyond the top of the trim, and then glued them to the trim using some construction adhesive (the trim is only 3/8" thick, and so not really viable to screw into), clamped it down and left it overnight. I then just used the double-sided tape to attach the metal bracks to the track, and then mounted the track+trim and it looks just like the rest of the doors.