Ok, here’s the biggest problem, and how I would solve it:
One “s” is a short slur, but ss is a ligature: a key combination that returns a different glyph (in this case, a longer slur). I created the following ligatures:
s_s.liga
s_s_s.liga
s_s… up to seven keystrokes. I made each one 200 units longer than the last. All slur ligatures are zero-width, meaning they don’t advance the cursor because they don’t take up any space. The slurs look bad because I just grabbed and stretched them in the font program. If you wanted to do it properly, your best bet would be to create some slurs of increasing lengths in Dorico and snip them as SVGs using graphic slices:
Then you could import them into the font program to use as glyphs for each of your ligatures.