I understand this need…I wrote a piece of software to do the “slow down but not change pitch” so that I could transcribe jazz solos. This was maybe 10 years ago, but I used someone else’s DSP implementation so I don’t know exactly how it worked inside.
That one is a little bit complicated because it messes with the definition of time.
For pitch shifting, I’ve always adjusted it at the instrument. That does not work for everyone, though…