Some cats are just thin. We have a 19 year old Blue Russian mix that we can’t get over 7 pounds no matter what. Blood, health is fine, just a very active cat (for a cat ;).
Just sayin’.
My cat is just naturally thin too. I used to feed her a full bowl 3 times a day along with snacks, which only amounted to a bit of a saggy belly and tons of poop in the litter box. I dropped her down to 2 half bowls of food a day and she still looks the same 6 months later (saggy belly slightly smaller).
She, like your cat, is also very active.