I completed my firest Tier 1 Mastery yesterday (Mickle). Sorry for the newbie question, but was it my 'reward'? Did that suppose to have unlocked something?
You can change your main menu background for the lake you completed in. Just this so far pretty sure there will be more to come.
Yes, the splash screens are the level 1 rewards. In the past(FSW), they had 3 levels for each Mastery. Each level obviously more challenging but, they tied them to in game apparels; special shirts, hats, etc. They haven't mentioned what would be the rewards this time around. I'm not sure how good the rewards will be seeing we lost over half the sponsors, equipment & clothes that were present in FSW?