First I can't offer projections from current eanring to future earnings. I don't know what they are making currently, what percentage of players are spending money. I don't have any of that data.
No the unlocking ideas are not off the table. You need that element.
I'll explain.
In addition to offering various methods to buy the right to play certain courses, be it single play, single purchase or bundle, you also must leave the option to unlock all courses through regular game play. Players who choose to purchase courses may either not have the skill to unlock them or do not have the patience. Some players will not care to unlock them. But you need those players to.
Even players who spend nothing in a free to play game have value. They are the opponents for those that do spend money. You need people to play with, and to keep plenty of people playing you must make everything you have for sale available through the course of play. It is of course not going to be easy for them to unlock courses. They will still try though because it is a goal to work toward. It is the illusion of no payment barriers to content. This will keep a large player base.
Now how much to charge. I am not sure, it depends on how many courses there are. The more courses available, the lower I would charge per course. Not too low though. I would make the price per course rather expensive and encourage players to purchase courses in bundles at a discount. I would also offer special deals around holidays for some of the more sought after courses.
The money sink. Right now I am guessing the bread and butter for this game is of course ads, but after that there is golf balls. I base this assumption on what I have spent playing this game. I think that will have to stay for now. It is an established practice, so there is no need to abolish it. However, I think that if there was 60 more courses to market, that would become the number one earner over ads and balls at least in the short term. Selling unlimited access to courses makes it a limited commodity, so a sales decline will eventually occur. I would use the initial surge of capital as an investment in advertising the game. That advertising will bring in more players and more profits.
Well that sounds like a good plan to me, but who am I.
Edit: You will also need a new match making system. (It is needed anyway).
Also if there is sixty courses it might be better to have unlocks of courses happen after a certain number of ranked plays on courses. Some courses should be reserved to unlock at higher tiers, but maybe 75% should be unlocked through just playing lots of rounds.