Google pointed me to another popular open source package, Maxima. It looks a bit rudimentary (command like interface), but formulas can actually be very easily exported to latex with the tex command. Here is a simple example:
(%i14) D(x):=sqrt((lambda-rho*eta*x)^2+(-x^2+x)*eta^2);
2 2 2
(%o14) D(x) := sqrt((lambda - rho eta x) + (- x + x) eta )
(%i15) G(x) := (lambda - rho*eta*x - D(x))/(lambda - rho*eta*x +D(x));
lambda - rho eta x - D(x)
(%o15) G(x) := -------------------------
lambda - rho eta x + D(x)
(%i16) tex(taylor((1-exp(-t*D(x)))/(1-G(x)*exp(-t*D(x)))*(lambda - rho*eta*x - D(x)),x,0,3));
−(etλ−1)η2x2etλλ+((4etλtη3ρ+(4(etλ)2−4etλ)η2)λ2+((−4(etλ)2+4etλ)η3ρ−2etλtη4)λ+((etλ)2−1)η4)x28(etλ)2λ3+((8(etλ)2t2η4ρ2−16(etλ)2tη3ρ)λ4+(16(etλ)2tη4ρ2+(−8(etλ)2t2η5+(16(etλ)3−16(etλ)2)η3)ρ+16(etλ)2tη4)λ3+((−16(etλ)3+16(etλ)2)η4ρ2+(−16(etλ)2−8etλ)tη5ρ+2(etλ)2t2η6+(−8(etλ)3+8etλ)η4)λ2+((12(etλ)3−12etλ)η5ρ+(2(etλ)2+4etλ)tη6)λ+(−2(etλ)3−(etλ)2+2etλ+1)η6)x332(etλ)3λ5+⋯
Regarding Taylor expansion, there seems to be quite a few options possible, but I found that the default expansion was already relatively easy to read. XCas produced less readable expansions, or just failed.
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