One option is to add a second toilet next to the existing one
. He might use one for hoarding and one for peeing. Or he might use both for both lol. There is one behavioural thing hamsters do and that is pee on their hoard if it has been removed before or disturbed. So by peeing on it they think (I think) it deters people from stealing it. Sometimes they just start doing it though and hamsters vary. I usually find most food is hoarded under the nest but special things get put in the toilet! Walnuts eg. Also sometimes they are running out of room in the main hosts so start hoarding somewhere else.
One thing I have noticed is that they cleverly seem to try and preserve their hoard and I have often found a lot of sand mixed in with the main hoard u see the nest - even when the sand potty is nowhere near it. So they must pouch it and take it there. Maybe and inbuilt thing to preserve their hoards - bury them in sand. That doesn’t account for peeing on a hoard which doesn’t do it any good! But designed to put people (or things) off stealing it.
It sounds like your hammy is protecting his hoard by peeing away from it.
One thing I used to do if they start hoarding in the potty is - take the food out and put it next to the potty - and keep doing this. They sometimes then learn to put the hoard there instead. I’d do it so the hamster could see what you’re doing (and talk to him at the same time and explain what I was doing!). It has worked sometimes.
I think some hansters thing the potty is just a useful hoarding receptacle - ours sometimes puts food in a ceramic hide too. So by giving him two potties it might just work.
As for the detolf - I also think it would be messy and difficult to clean with a sand area so I’d try a bowl.