Assume and Simplify not simplifying fully

I'm afraid there's not much you can do about it.
The "assume" modifier has always been a bit problematic and has often not worked as desired.
Apart from that, the symbolics in Mathcad and Prime is not at the level of Mathematica or Maple or MatLab with the current muPad implementation.
Different symbolic engines also 'simplify' differently. Prime 6 was the only version in which two engines were integrated - an ancient version of muPad (as it was also integrated in Mathcad 14 & 15 and Prime 1 to 5) and PTC's current in-house development based on friCAS/Axiom.
Here is a screenshot of Prime 6.

You can see that the new engine is still dragging along even two unnecessary absolute value functions - around sin(t) as well as around cos(t). In this respect, Prime 10 is already a small improvement. With PTC and Prime, you just have to be prepared for program development at glacial speed .... 😉

BTW, good old Mathcad 11 had a version of Maple integrated for symbolic calculations - one of the reasons that this version is considered by many long-standing Mathcad users as the best ever.

EDIT: Here some results of Prime 10

Here are some results of Mathcad 11/Maple:

This doesn't work:

Instead, you'd have to do:

But the assume apparently doesn't help. Other results are:

As demonstrated, it is hard to control the output of the symbolic processor.

But eventually:

Success!
Luc