I would use the 6061 round bar over 5052 since the 6061 is harder and stronger and tempered so it will likely take more torque with less distortion over the much softer and less stiff 50 series bars.
I had intended to get some drawing done to reply to an earlier exchange, unfortunately I have been so busy at my day job; time has been a premium.
I have included some old sketches here, to continue the running gear discussion and to give examples of ways that others have solved the many rudder and shaft design issues.
This image shows a welded rudder that was eventually cut to make it removable, the shaft allowance hole is show near the leading edge.
by adding this flange in the center of the rudder the two halves were recovered and in the future it could be removed more easily.
I have usually made the top shaft and rudder blades separate, and done the same with the pintle shaft or stub shaft at the bottom of the rudder. The idea is that a flange nearest the hull and at the top of the rudder isn't much drag, in fact there are rudder designs with plates top and bottom and I usually incorporated the flanges in those plates. Regardless if the top shaft is SS or aluminum this method will work, and the galvanic corrosion issues are usually defended against by plastic insulating gaskets, washers and if needed tubing sleeves on the bolts of one metal (SS) as they pass through the other (Al).
Here I show the bushing is bronze inside a steel welded shell for a steel boat. In the aluminum version you'd use an aluminum housing turned on the lathe to hold the plastic bushing and use separate 'floating' washers top and bottom to act against forces along the length of the shaft.
As regards the method of making a rudder pintle bearing for the bottom of the rudder, this method has worked for many power boats that I've been involved in building or repairing. In the image the labels were for a steel boat application and I've not edited that image.
The idea is to turn a large round of aluminum as the main welded on housing and this would be located at the aft end of the rudder pintle shoe bolted (or sometimes welded) to the bottom of the keel. The aluminum housing has a plastic insert that takes the side loads of the rudder's thrust and allows a water lubricated non-greased bushing or bearing point. This will work with a SS stub shaft or an aluminum shaft.
If and when the commercial fishing boats we built were aground or if they hit a bar or log while running we wanted to transmit the forces up the rudder shaft and blade without driving the pintle upward- so the rudder pintle shoe extended behind the keel was supported by the rudder AT THE CENTERLINE of the rudder and shaft.
Therefore there is a shoulder no the shaft/stub shaft/pintle shaft in order to have the SS washer push upward on the entire shaft and rudder blade. There was a corresponding shoulder on the rudder topshaft to accept this compressive loading, when, and if it ever happened.
So, this drawing isn't the exact model you'd build for a 25' er but it will give you and idea of one way to make these parts for your Titan.
Not shown are the rudder pintle shoe or keel extension that holds this bearing assembly to the keel and protects the bottom of the wheel and rudder from anything coming upward from below.
As regards the rudder stuffing box when you use aluminum for shafting. Aluminum will become roughened and begin to abrade more easily than SS so the traditional stuffing box can be modified to allow the less hard alloy to serve. This picture is just a look at the more or less traditional rudder stuffing box.
here is a close up cut away of that traditional box/gland/shaft seal and bearing showing the stuffing box 'rope' and how the top and bottom of the chamber will cause the packing to squeeze around the shaft when the top is tightened. That is where the wear on aluminum starts.
here is one way to avoid premature wear on softer alloy shafts. There is a plastic grease ring in the middle of the rope packing stack that will allow grease from the side mounted fitting to flow all the way around the shaft and into the shaft to packing interface along the shaft's surface inside the stuffing box.
This will allow the grease to be introduced into a very tight space so the 'shear' is much higher and therefore almost no pressure on the packing is needed to give a water tight seal and the shaft wear is reduced dramatically over using aluminum in a convention packing rope sealing application.
here is an example of a plastic ring turned to allow a galley of grease to flow around the outside and bored to allow the lube to flow into the inner surface and onto the aluminum shaft surface.
Carlos, I hope to have stimulated your thinking about how to apply these concepts on your boat. I'm also confident that you'll improve them as your own design ideas grow and I'm sorry not to have been responsive earlier.