I think having two extra feats above and beyond everyone else is pretty inherent and declarative of the class. And with those two extra feats above and beyond what anyone else gets, you have plenty of choice to focus on non-combat areas of specialization.
I think we are using different definitions of "declarative." I...can't honestly say I understand exactly how you mean it, but at least I can explain my usage. A "declarative" ability is something you, as a player, elect to "expend" or "use" or the like (e.g. you
declare its use). This need not
necessarily also mean that the character's behavior be identical, though the ideal case would be where the player's choice is mirrored, in the fiction, by a specific choice on the character's part. For example, Action Surge is a declarative ability, though as I've said elsewhere, it's heavily slanted toward combat-related uses and I remain skeptical about its uses in the other pillars. Essentially all "spells" are, by nature, declarative things. (If we're speaking very technically, each and every "attack" also would qualify as "declarative," but I'd call that a trivial case since it's a declarative ability shared by 100% of classes.)
Passing over the spells suggestion, since I doubt anyone (myself included) feels like having me re-rehash my distaste for resorting to spells (a distinctly "non-martial" mechanic) to achieve such ends...
Or if you don't like spells, go with
At 6th level you get the lucky feat
At 12th level you get the dungeon delver feat
Both two hugely impactful non-combat skills. Along with a lot of other choices to choose from.
There's nothing declarative about Dungeon Delver. It's entirely a set of passive bonuses, and since I consider trap
damage to be a combat thing, its primary benefits are combat-related. Discovering and disarming/subverting traps is more clearly exploration-related, but once it's actually doing the hurty things, it's effectively an enemy (or an enemy's attack, if you prefer). This is for the same reason that I consider, say, the Dragonborn elemental resistance a "combat" benefit, or the Barbarian's d12 Hit Die a "combat" benefit. Nothing about Dungeon Delver helps with discovering, disarming, or subverting a trap, unless you count "setting the trap off" as "discovering" it (which, I mean, sure you know it
was there, but it's not really "a trap" anymore once it's sprung).
The "advantage to find secret doors" benefit is especially hollow, because its biggest benefits come into play only when your DM is not very good at using secret doors. If a secret door is an essential plot element, that is if the story
cannot progress until it's found, then finding it absolutely should not be predicated on player skill, it should be part of the fiction, just like any other "plot hook"/"story-advancing" element.* If the door is merely extra loot, I hardly think having advantage on that check is particularly related to
any of the pillars, because treasure is treasure. And if the door is helpful without being essential, perhaps hiding a
helpful clue when the players already have enough to move on, there's a good chance they won't even think to look for it anyway.
Perhaps this is simply my inexperience with dungeon-delving (most of my campaigns with significant traps have spent more time in the "overworld," if you'll pardon the vaguely videogamey term, or one/two room buildings directly connected to the out-of-doors), but the only even remotely "declarative" bit--search for traps at "normal" rather than "slow" pace--seems of very limited utility to me. How often is one going to need the extra mile per hour when within a dungeon? When are there going to be so many traps that Normal speed is a problem in the "overworld"?
Lucky is...I'm honestly not sure HOW to feel about Lucky. It's unquestionably powerful, the equivalent of "stacking advantage." And it applies to any "ability check," sure. I'd call it rather a mixed bag, since that kind of powerful benefit, much like Action Surge, would feel wasteful if used on anything less than a *very* important check...and I find that the vast majority of nail-biting, "will-I-or-won't-I-make-it" checks occur in combat. The fact that two of the uses (your own attack rolls, or being attacked) are purely combat, and a third (saving throw) is
almost purely combat, doesn't help make it seem strongly non-combat oriented.
I'm also still stuck on: what's actually
Fightery about either of these things? Anyone can be supernaturally lucky. Anyone can know dungeons well. What's
Fightery about doing those things? The fact that you can do them while also being strong/quick? That's...really quite boring, if that's what "being a Fighter" means. I'm also left asking, why force me to choose between these benefits, and combat benefits? Why
allow people the option to have nothing special to do in 2/3 of the game's core, high-priority situations? That's a crappy choice to give people. I genuinely don't understand why it's so bad to have more than just "I can roll skill checks" or "I can compare [strike]size[/strike] *ahem*
combat stats with someone!" for the enormous number of things a "soldier"-type should be good at that aren't about the "make things die" part of fighting. You could even have a list--just like a list of fighting styles--so that people don't feel like they MUST accept a particular kind of flavor within the "Fighter" archetype.
Why is it such an abomination to have special Fighter actions...that are about all the
non-killing parts of being a "trained" (whether self-trained or not!) combatant?
*For example, if the meaning of the sigil on the escaped assassin's cloak is essential to moving the story forward, then that meaning should be provided to the players without depending on the whims of chance. That said, this automatic provision should only apply up to the point at which the story can definitely progress; the players don't necessarily need to be told that the sigil is faked, or that it has specific filigree details that point to the heir apparent, rather than the current head of the house. Another way to say this: details that add or remove
intrigue can be left to chance, but details which are essential for the players progressing on the course they've chosen (even if "progress" means "crap, we followed the wrong lead") should not be left to chance.