You're not locked into a side until pretty late, so you've got breathing room. The main story quest that kicks off Dawnguard is "Bloodline" - that's where you actually have to choose, and it doesn't pop until you've done a decent chunk of earlier stuff. Before that, you can do pretty much all the Dawnguard faction quests without committing to anything permanent.
The perks are legit worth it imo. The Dawnguard-specific perk tree has some solid stuff - there's one that boosts your damage against undead, another that lets you craft crossbows and bolts, and if you go heavy armor there's a perk that stacks armor rating when you're wearing the full Dawnguard set. The gear itself is solid early-to-mid game but you'll outscale it eventually unless you're doing a roleplay run. The crossbows are actually fun though, especially if you're modding - there's some decent crossbow overhauls on PC that make them feel less like just "bow but worse."
If you're not feeling the vampire stuff, just skip Harkon's questline entirely and stick with the Dawnguard side. You won't miss anything story-critical for the main game, and honestly the vampire perks are kind of a trap anyway unless you're specifically building around them. The Dawnguard content stands fine on its own.
Fair warning though - some of the later Dawnguard quests do have you fighting in vampire-heavy areas, so you'll still see plenty of them. But yeah, you can totally do the whole faction without ever joining Harkon or touching Serana's questline if you don't want to.
