After a string of failures, Aussie builders gave up on using bushings for weapon bearings. Back in the day, those failures were probably in bearings only doing ~ 5,000 RPM so at 10K or 15K, the failure rate will be magnified and the friction losses could mean that a larger motor is needed to get the target speed. I like bronze bushings too, I'm just quoting what my engineering books say about the RPM limitations.

Most 20mm ID ball bearings have an OD between 42 and 47mm, which makes fitting them in the hub of a small diameter disk a PITA. Perhaps a heavy duty needle roller bearing (or two) is the best alternative? They can take high RPM have a small diameter and are relatively cheap. The only drawback is that they have relatively low load performance, so they may need replacing after major impacts.

Here is an idea: find a bushing with the same OD as a needle roller bearing; if the bushing doesn't work out, you can just graduate to the roller bearing.