Midpoint arrows and shortening with the sep option

I have been experimenting with the TikZ sep arrow option, which allows to space arrows. I think that it can be hugely useful to also make midpoint arrows and to shorten paths. I have not yet seen anyone do this – maybe I have not looked closely enough.

Here I make some arrow distanced fist from one end and then the other end:

documentclass[tikz, border=1cm]{standalone}
usetikzlibrary {arrows.meta,bending}
begin{document}
begin{tikzpicture}
newcommand{mypath}{(0,0) to[out=90, in=-90] (4,2)}
draw [red, -{>[sep=0.5cm] Butt Cap[]}] mypath;
draw [green, yshift=-1cm, {Butt Cap[] Stealth[reversed, sep=0.5cm]}-] mypath;
draw [blue, yshift=-2cm,
   {Butt Cap[] 
   Stealth[reversed, sep=0.1cm]
   Stealth[reversed, sep=0.2cm]
   Stealth[reversed, sep=0.4cm]
   Stealth[reversed, sep=0.8cm]
   Stealth[reversed, sep=1.6cm]
   }-] mypath;
end{tikzpicture}
end{document}

I do not like Butt Cap[] to avoid an arrow in the end. Are there some notation for no arrow. This: ->- does not work. Maybe a empty or arrow[none] that I have overlooked in the manual?
Is there any way to give relative length to sep? – sep=0.5 does not give a midpoint arrow (the unit is apparently pt).

---

Here I use the sep to shorten without changing the path. Normal shorten will shorten tangentially from the end point – see the green path:

documentclass[tikz, border=1cm]{standalone}
usetikzlibrary {arrows.meta,bending}
begin{document}
begin{tikzpicture}
newcommand{mypath}{(0,0) to[out=90, in=-90] (4,2)}
draw [gray, thick] mypath;
draw [red, -{Butt Cap[sep=0.5cm]}] mypath;
draw [green, shorten >=0.5cm] mypath;
end{tikzpicture}
end{document}

Again, I do not like the Butt Cap arrow, as it overwrites(or are rather in addition to) the default line cap.

Edit: I am aware of many other midpoint arrow solutions e.g. here TikZ: How to draw an arrow in the middle of the line? , but none that uses sep.

documentclass[tikz, border=1cm]{standalone}
usetikzlibrary {arrows.meta,bending}
begin{document}
begin{tikzpicture}
newcommand{mypath}{(0,0) to[out=90, in=-90] (4,2)}
draw [red, -{>[sep=0.5cm] Butt Cap[]}] mypath;
draw [green, yshift=-1cm, {Butt Cap[] Stealth[reversed, sep=0.5cm]}-] mypath;
draw [blue, yshift=-2cm,
   {Butt Cap[] 
   Stealth[reversed, sep=0.1cm]
   Stealth[reversed, sep=0.2cm]
   Stealth[reversed, sep=0.4cm]
   Stealth[reversed, sep=0.8cm]
   Stealth[reversed, sep=1.6cm]
   }-] mypath;
begin{scope}[xshift=6cm,b/.tip={Butt Cap[]}]
  draw [red, -{>[sep=0.5cm]b}] mypath;
  draw [green, yshift=-1cm, {b Stealth[reversed, sep=0.5cm]}-] mypath;
  draw [blue, yshift=-2cm,
     {b 
     Stealth[reversed, sep=0.1cm]
     Stealth[reversed, sep=0.2cm]
     Stealth[reversed, sep=0.4cm]
     Stealth[reversed, sep=0.8cm]
     Stealth[reversed, sep=1.6cm]
     }-] mypath;
end{scope}
end{tikzpicture}
end{document}