I would adjust the rear brake lever down if it is locking up the rear wheel with normal applied pressure. I had to do that to mine, since it too would lock up very easily, particularly while practicing emergency stops. After adjustment, rear wheel lockups have not been an issue. I have to make a concerted effort to achieve a lockup now.

Did the same. Not happy with the excess travel, but it is safer.

19-inch front has a larger radius than the 18, all else being equal, so the speedo will be more accurate. U. S. of A. government and some others require optimism of motorcycle speedos by prohibiting pessimism.

Same brake will be less effective with larger rolling radius wheel and tire. That's just physics. CSC has already provided a nice solution.

Remember, some folks prefer the quicker handling 18-inch wheel for their commutes around and through town, so more will be happy with their choices.