I played
ror-nan in a recent Burn Bryte campaign, which is a bug swarm in a humanoid shape. In that that system your species more or less acts as another class you can buy abilities from, which I focused on. So I could send out a single bug as a spy, scatter myself to quickly explore a large area, and engage in body morphing shenanigans similar to Reed Richards/Plastic Man.
I think its important to decide what kind of mechanical hook you want the character to have, and how much of it comes from the physical form vs the class. Like, you can essentially just be any species refluffed if you don't mind the swarm not being front and center, or design a custom class for your character that incorporates learned skills and evolving species traits.
If you don't care much about expanding the swarm aspect, the
tortle works well, at least for hermit crabs that are more land based (some only breathe water, some actually aren't fully aquatic). You get the base AC from the suit of armor you're in, can retract in your shell, and the unarmed strike represents a swipe from many tiny pinchers. Hermit crabs are natural scavengers, so survival as a bonus skill fits well.
If you want a bit more swarm traits, you might talk to your DM about a series of feats, or swapping out (sub)class features for swarm stuff. Or go all in with the custom class.