The game being about "almost anything" is the issue
You said "in D&D you can try to do almost anything", and this is probably the underlying issue which should be addressed. Don't get me wrong, D&D is indeed a game of imagination. Unlike conventional board games, D&D does not limit players by a specific list of choices. The PHB makes accent on this several times:
Your character can do things not covered by the actions in this chapter.
You're not bound to those options, but they're a good starting point.
You just need to aspire to create, to have the courage of someone who is willing to build something and share it with others.
PHB also says that
The only limits to the actions you can attempt are your imagination and your character's ability scores.
Without context, this can be misinterpreted as "everything is possible". But it's not the case. The DM always sets the limits. The same paragraph says that explicitly:
... the DM tells you whether that action is possible
When you getting acquainted with your first game system, it's easy to fall for the illusion of its omnipotence. You think you can simulate any kind of situation, and the game system itself will magically make it playable, as long as you follow the rules. This is a common mistake made by new DMs. The game system itself doesn't have any magic. It's the DM who does the magic.
The rules don't explicitly say what you are supposed to do, but they give you a tool. And "if the only tool you have is a hammer, it's tempting to treat everything as if it were a nail". The Three Pillars of D&D are Combat, Exploration and Social interactions. Anything out of this scope (like, making contraptions) has little to no mechanical support. You can still do that, but the game system won't offer any help. Trying to squeeze D&D ruleset into solving problems out of its scope would be like unscrewing a screw with a hammer.
Every D&D game assumes a genre, and genres differ
Albeit DMG assumes different genres, a particular game should be about something specific. Being about anything is the same as being about nothing. 5e is a game of cooperative storytelling, and following different genres produce different stories — a gritty survival horror scenes won't suite a lighthearted comedy, and vice versa. Using a faraday cage stuffed with wool in real combat is like carrying a stepladder for fighting giants. The latter has strong comedy flavor which might not suit the rest of the game.
At the end of the day, it's about reaching a consensus
When we're talking about what is possible in 5e, we're mostly talking about what people believe is possible in this particular game world. The game system give us guidelines, but it doesn't actually say what is possible and what is not. For example, the woolen faraday cage should render the wearer almost completely immobile. You wear a gambeson, a plate armor, then you wrap yourself in thick layers of wool, then you cover this with copper plates which should be connected and grounded. I can't imagine anyone could even normally walk with all these things. But this is just my opinion. If the rest of the table disagree, I'd rather support the consensus.
For combat mechanics, stick to the published materials for now
Combat is a huge part of any D&D game. It is the main focus of the game system, it's also the primary subject of the game balancing. Adding "+3" to someone's sword too early could negatively affect other players' experience and make the game less enjoyable.
The same is true for damage resistance. An item which gives you resistance to multiple types of damage is very powerful in terms of game mechanics. Allowing mundane means to be as powerful as legendary artifacts opens a straight way to abuse.
If you're a new DM, I suggest you the following. Instead of homebrewing new things or introducing new mechanics, try reskinning things from existing published materials. And if you can't, just forbid the abuse: "I agree this sounds reasonable, but this will break the game, so I can't support this in terms of mechanics".
It's tempting to invent things on the fly — "I cover my daggers with dung to infect my enemies' wounds" — "wow, that's smart! now both daggers deal +2 damage and afflict Poisoned condition" — but this could derail quickly. Now all characters carry manure in their bags, which is not what the game was supposed to be about.
For encouraging creativeness you can freely apply Advantage/Disadvantage or other advantageous mechanics temporarily for situational things. Like, a well-timed Light cantrip blinded the goblins, but this worked only in that scene. This is known as "making a ruling" and is favored by the 5e.