Whoever told you to keep your tank at a salinity of 1.020 has zero clue and you should probably not listen to anything else they tell you.
I’m sorry you don’t have a ton of support from your parents with this hobby, but they probably understand that reef keeping can be a pretty massive undertaking and costly so maybe being cautious. I wish you the best of luck. It may be wise to make your next purchase or gift request to be an RODI system so you can make your own water. I heard you mention not having a ride to get water from the LFS to do water changes and that makes me wonder how you are managing top-off for evaporation as well.
This hobby is so much fun and I think it is awesome that you are getting into it at a young age. You sound like a real smart guy and if you stick with it, I’m sure you will have a stunning tank someday. You have an uphill battle though with probably little to no income and dependent on parents/relatives for rides to the LFS. Be patient and you will be 16 before you know it. I wish I had such productive and healthy hobbies when I was your age!