I have been playing MTGA for about a year. In that time I have heard Youtubers talk about how play drops off a month before a new set comes out. I have also noticed that with a month to go, the Mastery runs out, and you reward drops down to uncommon card rewards. I also notice that most of the uncommon cards that we get are ones that I already have 4 of. Remember that in the days of paper only, after drafts, most players would leave their commons and uncommons on the table because they didn't want them. After two month of being rewarded for playing with valued products between 500 Gold and 2,000 gold for every 1,000 XP, suddenly, your reward is of very little value. Does this add to the drop off of playing? Quite possibly. I know that I feel less inclined to play during this last month of the set.
My suggestion is that you up that ante to the end of the Mastery. You could give common or uncommon wild cards, or a few hundred gold, if you were worried about the cost of increased value. You could also just give 100 Gems or a Rare wild card or a pack from the current set. Something to keep the incentive going.
That's my suggestion. I hope you take it and improve my enjoyment of your game.