I modified an existing rack mount to move it back about 1.5 inches and provide clearance for my rear disk. This was made by doubling up two 2.5 inch L brackets and some cutting to the right length I wanted and screwing them to the rack mount bar. The two M6 screws connecting the L bracket to the original arms prevent the rack from pivoting and similar to other racks that use extender arms. It looks close to the brakes but there is at least 5mm clearance. About $6 in parts. It seems sound, stable, secure and should be adequate.
Before this, I had 3/8 spacers between the rack and frame to gain the clearance but I didn't really like that idea of possibly stressing the bolts and or the threads in the frame even though some racks have that type of design too.

aluminum scooter bits from the recycling bin will also work.
this mod has handled 25-pound loads for thousands of
miles of dirt and gravel.

Those L shaped brackets look like a good solution, I assume they are steel.

I have seen a lot of work arounds that look like they would promote metal fatigue, some of them are commercially available. But the two examples I see above look more reliable for the long term than some of the other ideas I have seen out there. Good thinking.

I've heard that moving your rear bags further back can cause wobble at higher speeds. What's your experience?

Just because you move your rack back doesn't mean you need to move your bags.

I have used both Surly and Tubus racks with no problems with discs or mounting (on a bunch of different disc bikes). I have never understood the use of fiddly bits when plenty of solutions already exist in premade racks that require nothing else. Maybe it's just me?

no effect at higher speeds, assuming you mean cruising down
mountains at 50 mph.

you may have read about wobble while standing and cranking
uphill. that may be, but with low 16" or 17" gear, no need to stand.

regardless, these mods really don't affect the position of the
weight that much. they move the rack up and/or back maybe
an inch. if you did notice it, you could move the bags forward
on the rack.

that bags in the back wobble anyway ... out of the saddle