SLIME is the best stuff. My rider was always going flat until I started to use Slime in it. I did have to add more Slime over the years but it lasted 15 or more years when I got rid of it.
That is a quality railing that you currently have. I would keep the rail and just have it refinished in black (or your choice) and replace only the balusters. Considerably cheaper and there is nothing wrong with the railing.
Do not use the "blade tilt". Put the crown on your miter saw table with the top down on the table as if it were the ceiling and the bottom edge along the back fence as the wall, holding at the angle it will be mounted at when installed. Then cut it at a 45 degree.
Since you're delving into the home repair venue, get a multi tool saw, very easy and safe to use and will become your go to saw for all cutting. Many different makers and for occasional use the cheap ones will work. Dremel Multi-Max MM20V 20V Variable Speed Cordless Oscillating Multi-Tool Kit...
Look for one of the Handyman companies and then verify their ability to reframe that corner.
They are out there it just takes some searching to find the right one.
There's a lot of info on YouTube. Avoid LG and Samsung. Do not get a door mounted ice/water in the refrigerator section. Only if it is mounted in a freezer door otherwise will be a problem.
Repairmen suggest you stay away from Samsung and LG as they are the most trouble prone. And never get ice/water in the refrigerator door (only in a freezer door)
I worked for an asphalt company doing this exact job. Hot asphalt is sprayed down and pea stone or chips put over it and rolled. It is a primary maintenance program in many New England town roads to this day. I even did my own driveway with it and it is still in place after 40+ years albeit...