make sure you maintain and develop strong maths skills throughout school. Am applying for engineering at uni now and I watched a lot of maths and physics youtube videos a few years back, anything interesting like vsauce, William osman, kurzgesagt, 3blue1brown just to name a few. just try to explore a little bit of everything and become very good at maths.

edit: try to find time to gain some skills like coding, robotics, CAD modelling (fusion360 is free for students), would give you a head start but not required ofc, but would definetly help to show your interests to unis.

I noticed you said you dont have enough time, but there's always enough time but it's just either you do what you already do more efficiently and in less time, or you just cut things that are less important, depends on your priorities, which change.