I would like to welcome you back to WGT after a long absence. You are right that you need to spend credits to get better equipment. You are also right in your desire not to spend cash (real money) but you are going to need to spend that other commodity that you have: TIME.
To get better, no matter what equipment that you use, you have to play. Every day is best. Take notes, written or mental. The more rounds you play, the more consistent your game will become All of this takes time which you may or may not have.
You also need to set a goal for getting credits. One of the best ways I have found is to use the WGT app on my smart phone. I do not use the app to play the game but I do get any where from 50 to 100 or more credits daily by watching videos as I do other things. Those other things include watching TV or using the porcelain throne. I also do the video carousel every day. There are only 7 each day ( there use to be more available) with 100 views worth 75 credits. Doesn't seem like much but by the end of a year it ends up as 1916 credits. That's $20 I did not spend. I do check surveys and try to do a few every month, but these are not my major source of credits. Also it is worth checking some of the offers by vendors who will give credits for purchases. I once bought a pair of shoes that I needed and got 1000 credits. Win, win. My goal is 150 credits / day which equates to about 1 ball / day based on the cost of the ball I use.
Only you can decide how much time or money to invest. Just know that if you enjoy the game and want to improve, it will take some kind of investment.