I kept a pleco for years (around 1 foot long) this way: took a flat piece of slate, blanched 8-12 some large spinach or kale leaves, rubber banded those to the slate, then put the slate on a place where it could stay relatively flat and still. My pleco would graze all the time until the leaves were all but gone. The heavier leaves (like kale) I blanched an extra 5-10 seconds longer than spinach as they are more fibrous. Blanching isn't necessary and probably wasn't the best thing for me to do, but it was a fast douse in the pot regardless. Today, I'd try it raw first and see if it eats it that way before blanching.
Plecos will eat other things as well (zucchini, cucumbers, sweet potatoes, lettuce, etc.) of course but a large amount of vegetation with some variety is what mine stayed healthy on. Never bugged anyone else and no one else in the tank wanted his food. My tank did have to have lots of WC as plecos, especially large well-fed plecos with veggie diets make lots of waste. Typically fed it one large slate of food and he grazed on that for 3-4 days, then he went hungry for a couple days. I also had wood in my tank which he seemed to graze on as well.
It seems to me that plecos in nature graze a lot and so trying to feed them just pellets as if they quickly stuff their bellies may not work too well. That's my experience anyway.