Currently I just use Visual Studio Code with a syntax highlighting plug-in for CA65 plus either a make file or shell script for building projects. From what I remember, modifying the plug-in for another assembler's syntax didn't look that hard, it's just a matter of editing some XML or JSON ...

I'm not sure from reading your code, coupled with my relative inexperience (what you are doing is more sophisticated than anything I've tried), but I think because you are exiting the VBI with RTI rather than jumping to the documented OS entry points, you are bypassing the OS routines to scan the ...

I'm at work without access to hobby stuff, so I cheated a bit by taking an existing ATR from a program I developed, deleted my program using Tools > Disk Explorer in Altirra then dragged and dropped your files into the image. I then renamed FREEWAY.COM to AUTORUN.SYS (again in Disk Explorer).

I can ...

As alluded to above, you need some form of DOS on the disk as well (or at least the A8 needs to have a DOS loaded). While the ATR is in SpartaDOS/X format, it doesn't actually contain SDX itself. Of course, you might be booting SDX from cartridge.

Attached (hopefully) is an ATR with Atari DOS 2.5 ...