Based on what I examined thru the One UI 3 update on the A51, We should get the One UI 5.1 update around Mid of May....
Galaxy A73 would get it first. Manifesting by February for the first Galaxy A series to have One UI 5.1
Thanks for sharing and keen interest with the latest One UI 5.1 roll out for your Samsung Galaxy A51, here.
Rollout and release times may vary depending on device version and model or service version, country/region, and network-dependent.
We thank you for your patience.