i don't have much insight for the learning curve for unreal but from someone who transitioned from unity to godot, it's not as hard as you might think it is, there are a lot of similarities like stated with the tweets below.
just jump in with learning about GD-script and the basics of nodes.
if you have knowledge in Python then GD-script won't be hard to transition to at all, and godot also has support for C# if you want to continue with the same language unity uses.
check this tweet out if you're confused with how entities, components, prefabs, etc. convert to godot https://twitter.com/reduzio/status/1702246568294769116.
another good tweet relating to switching from unity to godot https://twitter.com/reduzio/status/1702263225004306447.