One thing with having a stationary axle is that the rotating part has to be much thicker around the axle; with a 20mm axle, a 25mm OD bearing and at least 15mm of beater material around that and the beater will need a 55mm thick block of material, most of which ends up as waste. The advantage of a stationary axle is that it ties opposite sides of the bot together and ensures that bearings are aligned. Drum bots almost always use stationary axles for this reason, while beaters seldom do; they rely on the strength and rigidity of the beater to provide a 'virtual axle'.

With bearings, there are not many choices to use with titanium. One possible idea is to machine & polish a short steel tube and press fit that over a solid titanium axle - that would remove the galling problem with a bronze bushing. Apart from Mario, most vertical drum and beater spinners use ball bearings very successfully.

General design: Waaay back there was a builder called Daisy Robotics, who made a series of successful bots similar to yours. they used a short, wide drum but the concept is the same. To improve resistance to side attack he cut off the front corners at 45 degrees like this:



Turning a sharp 90 deg. corner into two 45 Deg corners adds a heap on strength, even if the weapon goes the full width of the bot.