I think it’s always good to be cautious with rabbits when you notice changes like that The soft poops that you are seeing, little balls joined together are cecal poops, they are normal. They look sort of like a blackberry with the way they are clustered together. See here: https://s-media-cache-ak0.pinimg.com/originals/e1/9c/b9/e19cb9d403771551bdec6f2a86cc9b95.jpg
You are correct that too many pellets, or too many other rich foods like greens can cause too many of these to be produced. Usually rabbits eat them but sometimes they may leave one especially if they are producing more than they need. Sometimes a particular pellet brand can also be the cause, some rabbits tolerate certain brands, but not other brands depending on how many extra ingredients they have. What brand had you been feeding?

It’s not necessary to see a vet for an extra cecal poop or two, however if it is meant to be more normal poop and is instead mushy, then it would probably do to keep away from pellets. Has Trim been to the vet since you got him? If not it’s probably worth just getting him a check up anyway and they’ll also be able to assess his weight for you.