privacy-wise, I started to feel really unsafe on reddit

Definitely. Reddit is trending toward a much more privacy-invasive posture as they lean into advertising and prepare for their IPO. I was a regular Lemmy user by the time they started blocking users behind VPNs, but that particular move was shockingly anti-consumer IMO.

I’m obsessed with things being open source and free and so lemmy just tickles me in the right way in that regard :).

Same :) I'd add "distributed" to that list, too-- It's comforting to know that the Lemmy ecosystem isn't one private entity that can make arbitrary decisions, or be bought out by any eccentric billionaires. Reddit has this army of mods and contributors that make it work, but as we saw in the API fiasco last year, the power dynamics there are really lopsided... Lemmy is much more empowering to the people who use it.

For specialised communities, some aren’t really active here. Ie. the community about the illness I suffer from has about 100 members here and is dead, while on reddit it has 50k members

Definitely a disadvantage, yeah. Reddit still has that scaled network effect that is hard to replicate. Lemmy is less lurker-friendly, but if you're willing to participate, I find that people actually show up. You can often post in those "dead" communities and find that there are lots of folks to talk to, they just haven't been posting there. It's much more like "if you want a warm fire, you gotta chop wood," here than on Reddit. Less convenient, but more satisfying IMO.

It was a little complicated choosing a node and signing up.

Yeah, that's a big barrier for people :\ Tech companies spend TONS on minimizing sign-up friction, but that "What is an instance? How do I pick one?" thing is a) frictionful, and b) intrinsic to any Fediverse system. I always tell people "don't overthink it, just pick one," but it's still a barrier.