Nice to see the update. Thanks!
If your water checks out and he has places to rest, the lighting is dim, the oxygen is plentiful, it could simply be its nature. Mine two strongly prefer pellets too but if I starve them 4-7 days they usually take fish.
Also, consider whether or not you are trying to overfeed your wels. They grow rather slowly and do not move around that much except maybe by night, so their metabolism is correspondingly slow. Perhaps your wels simply takes what it needs at his pace.
Jeez the government ruins all my dreams,whatever happened to the pursuit of happiness!?
In a nut shell, too many citizen are pretty keen on human rights, not enough care for human responsibilities... whilst these are two sides of the same coin. Without one of these there is simply no coin, that is no life. And having forgotten and abolished responsibilities, that's where we are all headed, towards no life. First figuratively. Next literally.
I do realize rights are fun, responsibilities are work. But we as a humankind are drifting away farther and farther from the common sense that to have fun, one needs to work.
IMHO, this is what lies at the very heart of all our problems without exception, from social and individual unhappiness, injustice, and wars to wrecking local ecosystems by irresponsible pet releases.