Don't fret the flex. As you point out it only happens when sprinting, and just about every good bike flexes then too. It'll also flex if you charge steep hills. Either way, it won't affect handling or safety because you're usually out of the saddle at that time, maybe rocking the frame, and so any sense of precise handling is already gone.

Ther are stiffer frames out there, but not necessarily better. In many ways race bikes are like sausages, you can enjoy them more if you don't know too much about them.

BTW- if you want to increase the BB stiffness against pedaling forces, you can do so by buttressing the seat tube in the inside. One way is to turn an old seat post down to the ID at the base of the seat tube and jam it into place, possibly with some bonding. Seat tubes are generally single butted with thicker walls at the bottom, but I don't know if your Ti tubing is.

Buttressing the tube is something you want to do right because you don't want to create a stress riser at the top of the buttress. In any case consider this only if after riding it a while you decide that it is in fact more flexible that desirable. (Based on riding, not experiments).