Still ideally we would solve this in another way then
switching to a threaded IRQ handler.

As the commit message of the mei-vsc fix mentions
the root cause of these errors is typically an interrupt
chip driver which uses IRQF_NO_THREAD disabling the auto
threading of all interrupt handlers in RT mode.

So the first question here would be to see if that flag is
used in the interrupt chip and if yes, is that flag really
necessary ?

This is very similar to the issue addressed in commit 24b176d8827d
("drm/msm/dsi: Remove spurious IRQF_ONESHOT flag").
The IRQF_ONESHOT flag is preventing forced threading here.

In irq_setup_forced_threading(), the conversion to threaded interrupts
is explicitly skipped if any of the IRQF_NO_THREAD, IRQF_PERCPU,
or IRQF_ONESHOT flags are present. In this case, IRQF_ONESHOT
appears to be the reason.

Regards,
giraffesnn