I only google resin bonded diamond wheel truing with silicon carbide wheel.
Most of sources had something in common, like using a brake or motor and make that truing wheel run delibrately slower than ir would if free running. I did not find much information about the grit of the truing wheel or grade, don't know how much it is of consequence.
One friend just used standard SIC wheel on small pedestal grinder on MDF board, flicked the switch on and off and brought it contact with diamond cup wheel and said that cup wheel got straight, but not necessary true, because of the manual (lack of) control. Said that it did not need "opening" after it, worked just fine.
Looks like front bearing of that Proxon spindle is finally shot, It is old and "lubricated for life" bearings generally don't last lifetime. Also can't judge if the front bearing has ZZ on small friction steel/elastic seal, probably something to consider when I order bearings for it.
I should give a call to local tool shop if they have opening sticks for diamond wheels.
I would like to hear other experiences, mine comes from forums and two tries. Not much of an experience.
Pekka