One thing you need to do is change your call
quadv(myfun,1,2000)
to
quadv(@myfun,1,2000)
To answer your side question:
Yes, if you create a function you can call it from the same routine. If you wrote the function in the form of a .m file, then if you are changing that .m file as you go, you need to "clear" the function before you can call it (or else you might get the old function.)
If you create a function handle in a routine, there is no problem calling upon the function in the routine. For example,
f = @(x) x.^2;
f(1:20)
To address your main question: it appears you are trying to use quadv() to come out with something that is intended to be a symbolic expression. You cannot do that: quadv() is strictly for numeric expressions.
Have you considered using int() ?
syms x2
int(myfun(x2), x2, 1, 2000)
