Doesn’t exist for the most part. At least not with good usability.
Exit relays: limited. You can change circuit using arm (Control and Monitor Tor), but it does not guarantee to change the exit relay.
Entry / exit relays: Afaik you can construct your own circuit using Tor’s control protocol and a custom Tor controller. Needs some programming skills. It’s “difficult” (I’d say 4/10 as per scale Frequently Asked Questions - Whonix ™ FAQ).
Circuit length, afaik it’s indeed just a compile time feature. Might be accessible through Tor’s control protocol. Dunno.