Not a terrible idea at all. I placed two mirrors across each other in our small bathroom. The mirrors are not of the same width but are the same design and at the same height to maintain simplicity. One narrow mirror above the basin, same width as the basin and tall enough so that our tallest family member at 2m can see their face. The WC also sits next to the basin but I did not want to extend the mirror from the basin. The wall space above the WC allowed for decorative items, eg artwork; narrow display shelf to add interest and texture. Directly opposite the basin and WC is a wide cabinet, and a large mirror to cover that width of cabinet and at the same height as the basin mirror. This mirror also reflects the artwork above the WC. It reflects so much more light and creates a wonderful feeling of space.

I have a power point on the wall above the vanity and between it and the bath but I don't plan to have anything plugged in permanently or ready to use even though I could, as I have a hob at the end of the bath which is over 700mm wide so the bath water is quite a distance from the powerpoint. There appeared to be no issues with putting it where I did but to my knowledge plenty of bathrooms have a powerpoint beside the vanity but based on your layout, I would at least put it on the shower side assuming you have a screen there.

How big are the floor tiles? It may look unbalanced with a more delicate tile in the floor and the 600x300 on the wall. Are you able to continue the flooring tile up at least one wall for cohesiveness.

IMO, Adding a full-length mirror next to the bathroom door is an excellent idea and not strange at all. In fact, it's a practical addition that can make the bathroom feel more spacious while also providing a convenient spot for checking outfits or doing hair and makeup. I agree with @dreamer's suggestion that a walk-in wet room is a great option for combining a bathtub and shower in one area. If space is limited, a shower over the bath can also be a functional solution. As the bathroom will be used by three teenage girls, it may be a good idea to consider a bigger vanity with a double basin to make it more convenient and accessible for them. This can help reduce delays in the morning rush to get ready for the day.