A bit of a bump. I really like the idea of making new trap monsters. I'm testing Big Shield Gardna in goat format these days, and it made me think about this card again and what it'd be like in a balanced format. When I saw it last week, I was impressed, but already felt like it was just a bit over the top. I can articulate my view now.
I'm not generally inclined to call defensive cards unbalanced, but I feel like this card can do too much at too little cost. You mentioned Big Shield Gardna, which is a perfect example of good card design. I want to compare that card in the context where it is being used: goat format.
BSG mainly does two things: counter Nobleman of Crossout, and provide a decent defense. NoC is so prominent in goat format (most people main two) that maining a dedicated counter is worthwhile. But it had a pretty specific goal. People want to set Magician of Faith / Magical Merchant / Dekoichi, but if the opponent reads it, they can NoC it and then all your copies of the card are gone. So what you can do is set another monster you don't mind losing to bait out NoC, and then set your flip monster later on.
So, technically speaking, the use of BSG isn't even to counter NoC banishing your flip monsters; I just explained how do that with any other LV4 or lower monster. The point of BSG is that when you bluff a MoF, instead of sacrificing another monster to NoC, you get to keep a monster, netting you card advantage. What you'll be left with is the other component I mentioned: decent defense. And even that it doesn't do perfectly. It has remarkable DEF, but the second attacker will kill BSG because it gets switched to ATK mode. A monster with 2000 DEF that stays in def mode would often be a better defense.
So it doesn't sound too impressive, yet it's still a worthwhile pick. Simply getting a +1 when your opponent uses a card they main two copies of is valuable enough.
Now compare this to Spying Through the Cracks. It doesn't counter any one specific card, but the activation of any monster effect (including hand traps, as was pointed out). This is roughly a third of the cards that would be played. This shifts the design from a passive defense to an active interruption of the opponent's game flow. That can get irritating. I hoped to see a monster trap that's triggered by something specific your opponent does, not that's activated almost entirely at will. That's why I feel like this card, despite using the same mechanism as BSG, is more comparable to trap cards that negate monster effects. And that kind of takes something away from the concept: why is it a monster in the first place, if it behaves like a regular trap card?
There are two well-designed trap cards in specific I want to compare it with: Breakthrough Skill and Grand Horn of Heaven.
(The latter doesn't negate effects, but it's similar in that it also negates a very widespread game mechanism, namely Special Summons.)
These cards have something in common: they're both -1s in most cases. BS doesn't destroy the monster (but you do lose that trap), and GHoH destroys the monster, but the opponent gets to draw a card as compensation. This is a fair way of balancing out cards that can negate something so common. What does STtC do after being triggered? It stays on the field, remains there as a barrier for that turn, and you can reuse it if you can protect it. You don't lose anything and your opponent doesn't get compensated. GHoH is especially similar in that it also slows the opponent down by ending the Main Phase, but that happens once only, there's no reuse.
I admit that, in terms of what you're left with after triggering, it's actually not that unbalanced. It's mostly its interrupting behaviour that disappoints me. And I also have a hunch why it is that way. Modern yugioh features so many monster effects, oftentimes broken, that one'd want to slow it all down with universal monster negation. In that sense, this card feels like a reaction to an unbalanced meta in an attempt to make it less extreme. But a skillful format comes not from countering unbalanced cards with other unbalanced cards, but rather from not having unbalanced cards in the pool in the first place. I feel like, in a truly balanced format, a card like this wouldn't be necessary and we'd be content with having our trap monsters be more passive forces.
But props for the idea of reviving trap monsters, it's something I might give a try myself. Also, I hope I'm not coming off as too negative here. I actually think the card has much better design than most others I see. It's just that I have rather high standards. There's also the issue of not knowing in what kind of format the card would be played. DPR is a nice effort to solve that, but I haven't checked most of the other cards. I'm comparing it to something of the same level as Goat format.