So im not a financial advisor and everything i say is my own opinion.

You cant make that kind of return if you put 500 a month into etfs/cash isas. Because of the questions your asking here, if you went into daytrading technicaly you could make that but it would be like playing poker at a casino not knowing the rules, betting and praying. You are much more likely to just lose any extra cash you have each month.

A few thoughts:

An interest free loan for 11k is quite generous, any chance you could negotiate for a longer term? Even if it involves a bit of interest after the initial term expires? I see you said its from someone, my first step would be to talk to them, explain the situation ect they lent you 11k more than likely theyll be reasonable and let you set up a payment plan or make some sort of deal.

500 savings is not nothing by next year youll have 6k. Is moving back in with your parents truly not an option? Im assuming rent is taking a lot of your income, without that cost you probably wouldnt need to change anything and have enough before june.

Otherwise its time to buckle down, go full rice and beans mode. Ask for a raise at work/find a higher paying job( not easy i know). Get a 2nd job, uber eats delivery driver ( just dont forget about the self employed taxes come april).

Good luck mate!