DE bit, I believe, is unique to frame-relay, although recall ATM's CLP bit serves a similar purpose. Neither provides direct feedback for flow control purposes, although dropping the frame/cell/packet may. Either might be explicity set (I believe on some equipment) to indicate lower priority traffic that's intentionally being sent at a rate higher than what's contracted with the cloud vendor.
FECN/BECN can be used on the router with an adaptive rate shaper.
ECN is a host-to-host flow management scheme which isn't really intended to set flow rates on routers. Many host stacks, I believe, don't support it.
PS:
If you're having an issue with some heavy bandwidth flows adversely impacting other light bandwidth flows on frame-relay, you might be better served with other QoS features.