@CMack6 you may be luck. From reading a C9300's QoS documentation, shaping appears to be supported.
The Catalyst 9Ks have interesting QoS support (supporting documentation, also better than it was years ago).
What I'm going to suggest, provides better visibility into your uplink's 300 Mbps, and perhaps provides more buffers for bursts. Note, without my working with your device, interactively, it would take a long, long (long) time to try to find fully effective QoS mitigation, if even possible. (I.e. forums don't allow fast turn around on analysis of what your device is doing, and improvement, if any, with QoS changes.)
Anyway, studying your device's QoS features, the following may help:
qos share-buffer
qos queue-softmax-multiplier 1200
!NB: unsure where the shaped packets will be queued, at parent or child or both.
!What I hope the following does is define only one active queue for your
!interface (well possibly two, parent and child), and maximize buffer allocations
!for that/those queue/queues, using both "hard" and "soft" buffers.
!If you issues is sustained congestion, buffer tuning alone, won't make
!much of a difference.
policy-map ParentShaper
class class-default
shape average 300000000 !may need to set up to about 15% less if shaper doesn't also count L2 overhead
priority level 2
queue-buffers ratio 100
service-policy ChildQoS
policy-map ChildQoS
class class-default
priority level 2
queue-buffers ratio 100
interface gig # !the "uplink"
service-policy output ParentShaper
