Ooh I love this question.
Exploration is not niche. It’s definitely a popular profession but it may have some sort of niche parts to it. Every new player guide and streamer I see says “get into exploration, its the best ISK for a new player” and while this is somewhat true, it’s completely saturated as a “career”. Lets break it down.
Anyone who’s been playing a month or so can dive into hacking and archeology, so that’s not what you’re looking for.
DED sites in highsec are also popular and highly contested. You can “generate” private 5/10s by doing dens and getting escalations in other ways, but again, there’s nothing special about 5/10s, or any of the other unrated sites.
Sleeper Caches are definitely niche and they show up in highsec. They’re sort of rare so they’re less of a “career” and more of a side activity. They require high scanning skills and a mix of tankiness and hacking/archeologying. There’s also Ghost Sites (anomalies, not technically exploration), but again… they’re rare, so it will be hard to carve out an exploration niche in highsec.
In lowsec you have 6/10s, which less people do because they’re afraid to go into lowsec but not quite niche. You’d need a t3 or battleship for these. There are also higher level sleeper caches (rare) and Besieged Covert Research Facilities (anomaly, not technically exploration). Besieged sites aren’t niche but they require a decent amount of sp so I’d say less people do them.
In Null you have your high ranked DEDsites, which are heavily run by folks and not remotely niche.
So basically, there are very few niches within exploration aside from sleeper caches and arguably 6/10s.
Here are some things off the top of my head that are actually niche, most of which require a lot of SP:
-High class wormhole escalations in a dread
-Level 5 missions
-Level 4 pirate burner missions in NPC nullsec
-Level 4 multiboxing mining missions in low/null
-Solo moon mining in low/null
-Solo customs office ownership
-Hunting officer spawns
-Farming drifters with a death star
-Running Tier 5 abyss’s
Thats all I can think of at the moment.