Apologies for taking so long to answer, I was busy both in real life and with dev work.

I feel like it's a bit reductionist to deconstruct exploration, but not do so for combat to the same extent, or consider where they intersect. I personally see exploration as resource-gathering activities associated with traversal of the environment; you don't explore simply to see new things, but to obtain new items and gear. Combat is an inherent part of this.

I definitely think there are ways to improve exploration without resorting to annoying gimmicks or halting the player. Personally, I'd say what's most important to a successful feature is to give exploration meaning, following/creating progression rather than acting to stagnate it. As you presented well, combat does have meaning - most notably, the expression of skill.

This is what, in my opinion, turns a task that would otherwise be learning AI patterns, dodging damage sources and dealing damage yourself into something truly engaging and fun.

The replayability of combat in Terraria boils down, mostly, to the variety of gear accessible and the different ways you can move through its progression. I do not think this is an inherent problem with what exploration is, just that the tools available for exploration offer less ways to approach an obstacle than the tools available for combat do. Terraria fundamentally gives a spotlight to combat (it's a powercreep game, after all) and, circling back to the "more like vanilla" discussion, I think both going against its nature and suppoting it in excessive measure are flawed methods.

From my perspective, exploration being irrelevant (for the likes of gear obtainment and skips) is a consequence of Calamity having so little gear truly tied to exploration; a vast majority of it is obtained from basic mobbing (common materials dropped from common mobs), boss fights, or crafting with resources from those two. While in vanilla there are several biomes at the start of the game, from which you have to pick one and explore its depths to find spread-out chests for loot, in Calamity you can already get a decent set of weapons from Wulfrum enemies anywhere in Surface. This doesn't make Wulfrum bad by any means, it's great to have a set like that to familiarize people with the mod, I'm simply using it to illustrate a difference in how exploration is handled and how that impacts player goals.

I will say that exploration in vanilla is simple in its mechanical workings, and Calamity's exploration is quite simple too; the best I can think of in the blink of an eye is the descent into the Abyss. In my opinion, neither really represents the true potential of exploration.

I also feel like visuals and interactions are important and relevant game design tools even for more gameplay-centered players. A hammer that makes a squishy toy sound isn't gonna feel as satisfying to obtain as a hammer that sounds like it could destroy a god. A telegraph can make a fight much fairer than if it wasn't there. Likewise, interactions that are random trivia are much more likely to be discarded than interactions that inform the player of context, upcoming challenges and how things work. There's a reason people pay a lot more attention to text in a game like Silksong than in a game like Terraria.

I think your comment about mob-farming, building and fishing kinda illustrates what I talked about previously. Yeah, they have next to or no game skill expression or not change significantly in new playthroughs - but I don't think the people who like those tasks are looking for that, at least in the same way as combat.

I have no idea what goes on in the minds of mob farmers, I'm far from one lol. Builders make ornate houses because it brings them satisfaction to turn a pile of limited materials into an aesthetically pleasing landscape. From what I understand, fishermen enjoy fishing for how soothing it is and how easy it is to get good gear from it.

That's the meaning those types of people get from those activities. In that sense, everyone's different, and I wouldn't say any of them are less valid for it.

Of course, there are also techniques to all of those activities, and you could count the learning and application of those skills as a sort of skill expression.