The computer crashing after a few seconds has been fixed indeed (was caused by a faulty connection of C7). But there's still 1 problem remaining.
KJoy does not seem to work at all - #1f reads all 255. I could get that semi-fixed by adjusting R11 to 1MOhm, which allows me to randomly use the joystick (#1f reads 0 or 255), but this also makes the machine a good bit more unstable, since it sometimes would give me a black screen with white border and random disk access when powering up.

Using /IORQ instead of /OIRQ to drive the joystick interface allows me to use the joystick without any issue (#1f all 0 and reacting to input) but eliminates TRDOS (listing a disk gives me a *no disk* error)

Any ideas what could be wrong there?