R75/6 straight line stability

[ME 109]

Yep, pump those tyres up. I run about 35/35
If there's still a problem, then fork alignment is next.

Edit, based on running new tyres.

Since the tires are new, after pressure, my next check would be tire mounting: a tire not fully seated on the bead. Much more likely with tubeless, but can happen with tube sometimes as well. After that check, THEN I'd go alignment next.

A tyre not fully seated at 60-75 will feel like a jackhammer, but with sufficient air pressure should not "squirm" ime.
Squirming is flat tyre territory , or melted bitumen, or that rubbery shit they put in alpine road cracks.
Try home mounting a pilot active on a rear snowflake. They can be very difficult to fully seat the bead, even with a tube.
And a big compressor.
Breaking the bead is also not easy.
Less than ideal actually for a roadside repair if you run tubes.

[ME 109]

How are your rear splines?

No affect on rear wheel wobble from splines.
Wheel bearings, spun wheel bearings or swing arm bearings affect wobble.

[Abstruse1]

I didn't inspect them carefully, but didn't notice any damage. I'll get the bike out today (I hope!) for a test with the higher tire pressure.

[Abstruse1]

Aired tires up as suggested. Got out for a brief ride yesterday, abt 10 mi at speed. Handling seemed fine. Stay tuned.