My advice (pls. don't blame me if I might not be 100% on path) for your money:
1) Invest in your arena to reach 19K - 20K spectator place asap
2) Invest in your arena to reach 19K - 20K spectator place asap
3) Invest in your arena to reach 19K - 20K spectator place asap - not to believe, but it is the most important thing :)
After you reach that - next step merch store lvl1; better lvl2 or lvl 3, depending on your possibilities - here also to help to increase your passive income.
Than I would go for the arena design - a highly personal decision from my point of view. As i plan to stay till my RL-retirement in BB, I will go for sure for the 4M arena as it gives the highest return in the long run. If you also can/ want to spend 4M is - as mentioned before - a very personal decision of you.
After you activate all possible active and passive income possibilities for your team, only than I would recommend to start to foster team streghth and team developement possibilities like training court and gym. Same time, I would try to keep my roster closely above the required league salary floor to avoid unneded expenses.
This in fact is more or less also my personal short to mid term plan for my teams finances :)