That's all really, I find playing online seasons and TOTW challenge generates the most coins. For example, on average you get 500 coins for an online game, 1 season = 10 games so that's 5000 coins straight away. Then add in bonus coins for completing the season, lets say 5500 on average. That's 10500 coins after 10 games.
So yeah, I'd say you're doing it the best way, also make sure to play TOTW challenge on at least professional difficulty to get another 600 coins at least. Just stick at it and it shouldn't take you long to get where you want to be coin-wise!
EDIT: Just remembered a YouTube video I watched, it was saying something about buying cheap gold players for 300 or less and selling them on for a 50-100 coin profit each time. It was recommended buying pacey players in particular as they go for more. Sounds like a really slow method but if you're patient I guess it could work. I haven't tried it personally so I can't say what I think of it.
Will look at doing the Gold Player buying/selling, have all the time in the world at work