If possible, the best defense is learning about the topics before they become controversial. Also, don't jump directly into the controversial part, but learn the fundamentals of the field first.

For example, the actual reason why I don't believe in homeopathy is not because I found the "anti-homeopathy" side more convincing in the debate than the "pro-homeopathy" side. I actually suspect that most doctors do not understand half of the stuff they do, and that a large part of medical best practice is simply placebo effect.

Instead, it is because at the elementary school I learned about atoms. How water is composed of molecules of H2O, how being liquid means that those molecules bounce around randomly. Homeopathy working would require this model to be completely wrong. But that model doesn't seem to be made by someone with an anti-homeopathy agenda, because it is used in so many unrelated contexts. Therefore, homeopathy is wrong.

Another example, evolution. Instead of jumping to the deep end (trying to make My Own Opinion™ on whether something could or couldn't have evolved, without actually understanding how biology works), I start simple: Things are made out of atoms, yes or no? Atoms make molecules, yes or no? DNA is a molecule, yes or no? DNA encodes the traits of the organism, yes or no? (Don't just say "yes", think about it. How specifically can different cells do different functions if all of them have the same DNA?) Is it plausible that sometimes the DNA copies incorrectly? It is plausible that sometime an error in copying provides an advantage to the organism? (Is that a frequent or a rare thing to happen? How would the organism with the magically improved DNA reproduce with the ones that have the original DNA?) Etc. Now if someone claims that evolution is not real, I can ask them which specific step of this chain was wrong.

Similarly, there are some controversial(?) topics in mathematics. Instead of engaging with them directly, I have downloaded a few university textbooks on topics that are prerequisites for that, and I am going through them step by step, actually doing the exercises, and probably being way more paranoid about them than an average student. That will take a lot of time. But I believe it is a time better spent than trying to address the controversies directly without having solid fundamentals.

Learning the fundamentals requires a lot of time, but it is a limited amount of time; I assume that at some moment I will know enough to make an expert judgment. Merely debating the topics may require less time in short term, but it is an unending process; there will always be yet another clever argument or yet another internet debate. Also, by learning the field I can become a (kind of) expert on the field, but by debating a controversial topic I become only... an expert on debating this one specific topic, in best case.

One thing that works in your favor, epistemically, is that in democracy it is not necessary to convince everyone; you just need to convince the majority. Most people are kinda stupid, so the arguments designed to convince them are usually not too complicated. If you actually listen to both sides, it is often possible to find the mistake. (Don't bother telling them, no one is going to listen.) Sometimes you find that both sides believe stupid things. Not in the "truth is exactly in the middle" way, but sometimes one side is like 90% right but really strongly insists on the one absurdity they hold dear, which the other side is happy to expose... and then connect to their own absurdities. Basically, don't assume that just because two people are arguing, one of them has to be right; you might be observing two idiots arguing for two different wrong things.

My other heuristic is to have a list of people I trust and ask about their opinion. Explaining how to make such list would be quite complicated, but similarly to the "learn the fundamentals" advice, those people should be experts on something related to the controversy, but not solely on the controversy itself. (You want people good at modeling things, not just good at arguing for/against a side of a controversy.)

I think it's often the case that neither A nor B are true. Common opinions are shallow, often simplified and exaggerated or even entirely besides the point.
Now, you're asking what a good way to form opinions is, well, it depends on what you want.

Do you want to know which side you should vote for to bring the future towards the state that you want?
Do you want to figure out which side is the most correct?
Do you want to figure out the actual truth behind the political issue?
Do you want to hold an opinion which won't disrupt your social life too much or make you unpopular?

I expect that these four will bring you to different answers.

(While I think I understand the problem well, I can't promise that I have a good solution. Besides, it's subjective. Since the topic is controversial, any answer I give will be influenced by the very biases that we're potentially interested in avoiding)

By the way, personally, I don't care much what foreign actors (or team A and B) have to say about anything, so it's not a factor which makes a difference to me.

Edit: I should probably have submitted this as a comment and not an answer. Oh well, I will think up an answer if you respond.

The idea that either A, B or something in between has to be right is for many political issues wrong. It's possible that both A and B are wrong. I don't see why would would start with a different assumption.

For most issues, you are not required to have an opinion and it's often better to focus your energies on issues where you have unique insight or power to affect the issue than focusing on national level political issues where you have neither unique insight nor power to influence them in a meaningful way.

Foreign actors will attempt to push people on twitter/reddit/etc. towards either (1) or (5), even if the answer is really (3) for them. Everyone I interact with is either partially influenced by these actors or discusses their opinions with people who are influenced by these actors.

Why do you consider it better to be manipulated by domestic actors than foreign actors? Why does it matter whether the actors are foreign?