You are romanticising law to a degree, but it is to a degree that most students at your stage romanticise it. The day to day life of being a barrister is different depending on the area of law you practise in. Barristers who work in crime, family, personal injury, employment and commercial (to name five quite prominent and broad areas, but there are more and plenty of nuances within those areas) will have different day to day lives, and in many ways will use their skills in a slightly different way. Advocacy in the employment tribunal, for example, is quite different to the crown court, which is different again to the family court. But fundamentally, the role of a barrister is to advise and advocate for your client, which very often involves plenty of time in court. So to that extent, you are right.
The bit that you're not so much getting wrong as not appreciating is a bit about the practical realities of the role, and a bit more about different practice areas. It is extremely common, even up to the point where candidates are applying for pupillage, to see people motivated by representing the vulnerable and variations on that theme (I read the phrase "speak truth to power" multiple times every year in pupillage applications), and for candidates early on to focus on crime and human rights as the areas that naturally feed into that. In reality, you don't get to choose or clients or the people you prosecute. In a criminal practice, you may prosecute those who are innocent but will certainly very often defend those who are guilty. But you're not there to determine your client's guilt or innocence. Whether in crime or any other area, you're there to increase your client's chances of success and to secure the best outcome for them, whatever that may be. That will mean that, at times, your client is acquitted when you think they were guilty. At others (and these times are much worse), it will mean a client being convicted who you thought was innocent. The point is that you're the representative, not the decision maker, and you have to see your role through that lens.
The second point is about practice areas. I can't say I ever really see my role in the context of representing the vulnerable etc, but then I almost exclusively represent companies and institutions, albeit very often those who are defending spurious or weak claims. But my role is to advise those clients, draft documents and represent them in court in a way that maximises their prospects of a good outcome, and I find that role and that process to be extremely satisfying. It is about being a good advocate, but it is also about devising a strategy that you can execute to represent that client properly. The skills that go into that are skills that you develop over many years, but it is using those skills that make the job worthwhile for me, generally irrespective of the actual outcome, albeit winning is always a better feeling than losing.
That general approach applies to every practice area though, and your own view can too. You can represent vulnerable defendants accused of crimes, but why not employees who have been discriminated against or dismissed? Why not those who have been injured through no fault of their own? Why not parents involved in disputes over their children, or who face losing their children entirely due to local authority intervention? Why not parents of children with special educational needs who are struggling to secure the provision they need? The legal profession is so much wider than crime and human rights (the latter is actually extremely niche). Even if you are driven by altruistic motivations, it doesn't have to be those two practice areas.
In terms of practical advice, you actually don't need to do law now. You have committed to medicine in the short term, so the best approach may well be to do that course and see how you feel as you progress. You can convert to law at postgrad if that desire to be a barrister doesn't go away, or even once you've started your career in medicine, and as much as that will involve more time and possibly debt, it is a viable option further down the line. Taking a gap year and coming back to law at undergrad is another option, but one that you should make from as informed a position as possible. You may not be able to secure formal work experience with a barristers chambers in time to change your mind and take a different route, but you can always attend magistrates courts, crown courts and county courts, and just sit in the public gallery and watch. You may even be able to speak to some of the representatives. At the very least, you can go and a have a look at people doing the job to see how that makes you feel about it.