I am a parent myself (7 years old).
I would be happy and so willing to pay for a few months of subscription if my son wants to learn japanese. You wrote you go to Japan, so it’s doulbe worth it.
WK is not a magic bullet. I am still figuring out if this is the right thing to do or not (Level 9 currently).
However, it’s worth to try at least and if its not working for you, you’ll need to stop.
The best thing you can do is to ask your parents for advise. Tell them you think this is a valuable tool for your studies and you’d like to try. However, you might to stop with it when school is giving you headache. All halfway sane parents would say you are great kid to take care of the familys money and giving your trip, I would certainly pay for the tool.
In your case, i would not yet bother with lifetime subscriptions yet. These are for people who already decided this is the right tool (giving their life circumstances). If you are not so sure, you tell your parents exactly that: you don’t know yet and you would like to give it a try.
To sum up: talk to the people who invest in you. Explain pro and cons and listen. In the end you’ll have their advise and you’ll feel if this is a good advise or not.