I'm just going to flesh out what I said in my comment since you seem to have found it helpful. The short answer is, unfortunately, "it depends," since defensive typing is just one piece of the equation to consider while teambuilding, but I'll try to give you some general tips that apply to most formats and generations.
Tip 1: Consider how commonly used the type that you are weak to is offensively.
Unlike in an in-game setting where you can expect to encounter a very diverse array of Pokemon from every type, different Pokemon types generally do not enjoy relatively equal offensive or defensive usage in competitive play. Some types have moves that are especially desirable that other types do not have an equivalent version of. This means that you cannot look at type weaknesses as interchangeable, since one weakness might not be a big deal while another might be a major problem.
- An extreme example of this can be found in Generation 1, where the Bug type suffered from a terrible roster of Pokemon and moves. Combined with the fact that the only Ghost move was the 20 BP Lick, which was also glitched to have no effect instead of being super-effective, this resulted in Psychic-types having virtually no relevant defensive weaknesses. It's not a problem to have up to 3 Psychic types on the same team, since Bug is just not relevant.
- On the other hand, the Bug type has become extremely offensively relevant with the introduction of U-turn, for better or worse, which has had significant ramifications for Psychic-types. In many mainstream formats, having a shared Bug weakness across your team can leave you vulnerable to pivot-heavy opponents.
Tip 2: Consider who the common users of the type you're weak to are.
Is it a Dragon Dance sweeper that you can't prevent from setting up? Is it a bulky staller that you have no way of chipping down? Different Pokemon require different answers, so figuring out how to address a type weakness involves narrowing down your scope to identify who the main threats to your team currently are and then strategizing against those Pokemon in particular. It's hard to give a 1-dimensional answer like "add an Electric-type!" when typing is only one aspect of a Pokemon.
- Check out the usage rankings for the tier that you're playing to get a better idea of how likely it is to come across a certain threat. This is how you could "look at it statistically" when teambuilding—popular metagames will have data that tells you exactly what % of teams include a certain Pokemon. Here's an example of one.
Tip 3: Remember that Pokemon commonly carry moves that do not match their own typing.
This may seem obvious, but it's a common mistake for beginners to forget that looking at two Pokemon's types in isolation will not always give you an accurate picture of who will come out on top. For instance, both Groudon forms very commonly run Rock-type moves despite not being Rock-type, so a naïve line of thinking like "My Pokemon is Flying-type, so it'll be good against a Ground-type!" falls flat. Familiarity with type matchups isn't enough—you also need some familiarity with movepools and common sets.
- Going back to U-turn, part of the problem is that it has a massive learnset with numerous eligible users. This means that being weak to Bug isn't only a problem because of Bug-type Pokemon, who may or may not actually be relevant despite the singular move's dominance.
- As mentioned in the comment, Earthquake and Earth Power are very commonly run by all kinds of Pokemon who have access to them because of their excellent coverage. Ice-type Pokemon themselves are held back because Ice is a horrible defensive typing, but Ice is a great offensive typing because it can check Dragon-types, and Water-types often have access to Ice moves.
Tip 4: Build off of a core to avoid ending up with major shared weaknesses.
Another common beginner mistake is to pick 6 good Pokemon in isolation without considering how they relate to the other Pokemon on the team. This is often where gaps in coverage develop, and this is why good teambuilders will build off of a core. A core is a group of Pokemon that work well to cover each other's weaknesses so that in theory, you should have a way to respond to every threat out there.
You're right that all the potential solutions you thought of could be countered—any single, uncounterable strategy that worked all the time would probably be banned to keep the game balanced—so it's important to let your Pokemon rely on its teammates to stand up to situations that it can't deal with.
Here are some resources to start learning about building from a core:
Tip 5: Just battle more!
As you battle with your team, you'll gain firsthand experience that helps you understand what your team is good or bad at. Is there a single Pokemon that always wipes you out when it shows up? Does your team always seem to be slower than your opponent? Are there opposing strategies that you don't really understand or which were unexpected? Sometimes, you just need to get out there and experiment a bit to get a feel for how your team is doing. Metagames also evolve over time, so just playing helps you notice new trends and shifts.
