Think Supernova708 hit the nail on the head
This guy had a grey beard before the events of the story even started
His output is probably obscene, but that puts a lot of strain on your hands. He probably isn't making this request because he wants to work one day a week, but because he can only work one day a week
Edit: Honestly, rereading, I think its definitely something tied to overwork. There's this implication after he helped out the MC in the past with DragonLand, he just immediately jumped ship and worked somewhere else. Next chapter we're probably going to get a reveal that he now has a variety of injuries and conditions from that period of his life.
None of this being rocket science, but I just think its neat how cleanly this is set up in such a short chapter. We're shown he's good, prolific, old, married (he's got a ring on his finger), and at one point, very eager to work. Could be the opposite, he wants to be free to work on a variety of projects, but if that were the reason, I'd suspect he'd freely mention that to better negotiate his position and salary, like, 'hey, my time is literally so valuable I've got all these other people poaching me, if you want me to stick around more pay me more'. So its something he isn't comfortable mentioning, which means its probably something personal.
Im way over explaining this. But it feels very satisfying to put the pieces together for an approximation for what the reveal is going to be.