Either this guy is an idiot or there was a translation error:
"I want you to get five intermediate-level recovery potions. If you succeed, I will consider extending the deadline of the debt."
Consider means he could still say, "Nah, fuck you, give me the orphanage."
"Extend the deadline" means he still has to pay the whole debt back...
Looking for or creating those potions doesn't really benefit him at all.
He has some strength, so at the very least he could have forced them to accept monthly payments until he pays all the debt back. Then he could have made the potions and sold them in the market or to them and after a while the debt would be gone.
Killing them is even better. Killing the king could be tricky. A-Rank adventurers are like babies to him, but there might be S-Rank hoes that he has some trouble with or maybe there is a god on the king's side, some bs like that, so he might not be able to kill him (well, assassination would work, but taking on the full force of the kingdom's army wouldn't work.)
The only reason I see for dumbfuck MC to keep the orphanage going is for the promise the king made to him (not sure if it was an actual promise.) It was kinda like, "If you raise 10 people in the orphanage, I'll send you back to your world." But yeah, if the guy got to know some people or hired some trustworthy guards, he could leave the girls to go for some quick adventures and earn money that way. Or he could take his harem with him and make them strong. It's not like they have to be in the orphanage all the time, so leaving on an adventure for a few years and coming back with them as grown ups (or some random adults) could potentially pass as "raising" 10 children in the orphanage.