3) Anyone have experience listening to both the Titian and the Rythmic?
No, but I can tell you that Rythmik subs are very clean and "dry" sounding. I believe it is mostly due to the very low distortion above 40 Hz, the region that matters most for critical music listening. Distortion does rise below 40 Hz with sealed versions, but I have not found this to be an issue. I have not heard anything better. You can spend quite a bit more on subs without getting an improvement.
5) I would like to build the sub out of 1” MDF should that yield better performance compared to ¾”
1" MDF is very heavy, much harder to work with, especially if lifting a sheet onto a roof rack, then off. I can run a whole sheet of 3/4" MDF through a table saw with help, but wouldn't try with 1". Although usually I lay it flat for the first cut.
You won't find a lot of improvement. Another way to go is cheap formply, especially if you are veneering your box. Painting OTOH would be a pest.
Bracing cut outs. Those aren't critical. You must let the bass move through the box and cut outs have a slight impact on box volume. The point is to meet those goals with minimum compromise on strength. Circles generally are best.
Bracing is simple in principle. Each panel must touch 4 sides of the box, you then have cut outs for the driver and amp, then you cut circles out for the bits that remain. You don't have to carefully follow the cut outs.