You can in some cases tweak the spacing. But typst is not designed for this possibility. Math spacing is governed my math classes, and if it were configurable, you’d be able to set the spacing for each class (and their combinations).
The following is ad-hoc
#let math-symbol = $+$.body.func()
#show math.equation: set align(start)
$
f(x) &= x^2 + 3x + 2 \
$
#show math.equation: eq => {
show math-symbol.where(text: "+"): h(0.1em) + [+] + h(0.1em)
show math-symbol.where(text: "="): h(0.2em) + [=] + h(0.2em)
eq
}
$
f(x) &= x^2 + 3x + 2 \
$
