Thank you!

12k rpm is asking much, Chris @ LMS says it's good for about 10k rpm. Angular bearings are rated at 12k rpm, if you double up they drop to about 9600rpm and if you decide to fit a radial shaft seal their limit is a surface speed of 18m/s. At 40mm diameter (the lower part of the LMS spindle) that's about 8400rpm.

Still, my main concern is how much and by what means to preload the bearings. I hope I get to the bottom of this.