Song Surgeon is a great one for mp3's.

https://songsurgeon.com/features.html

You can do all you said: change tempo, change key, loop only certain sections. It allows one to change speed and maintain audio quality. Most really destroy the sound quality if you slow it down. Song Surgeon maintains the quality to really slow, almost to 50%!

You mentioned YouTube. To use Song Surgeon you will need to convert the mp4 to Mp3 to use it.

Other wise there is a sister product called Video Surgeon for MP4's. I use it just about every day.

Both are spectacular pieces of software.