I think metacam is a good idea. It's hard to tell when they have pain sometimes and it can keep them comfortable. I'm wondering if his bottle could be lowered so he can drink from it more easily. They do have quite bendy necks! And one of ours always used to drink sideways from his bottle quite comfortably.
In terms of diet I would keep it simple. Science selective has everything they need nutritionally - you can soak it with a bit of boiled water (just a tiny) but then leave it too cool till its soft.
My regime for an older hamster is
1) Soaked science selective in a separate small dish
2) Normal hamster mix in their usual dish (they still like to pouch and hoard it)
3) Babyfood - they love it and it adds to their enjoyment of eating
4) A pinch of hemp seeds and a pinch of linseeds on a baby food lid - they lick them up. Hemp seeds are supposed to contain all vitamins and minerals and they enjoy them.
If he can't have the veg at the moment he'll appreciate the baby food.
For extra little dishes I use the lids from the jars of baby food. So I'd have the normal hamster bowl with the usual muesli mix in so they can have normal behaviours.
One baby food lid with a few pellets of soaked science selective. One baby food lid with the hemp seeds and linseeds (one at each side - somedays they'll eat one but not the other). And one babyfood lid with a heaped teaspoon of baby food on.
For the baby food you'll need the 4 months ones as they usually don't contain things like spices, pepper and onions (which aren't good). Also best to avoid things with tomato.
There are usually two or three varieties that have simple ingredients - chicken and pumpkin used to be a favourite here. Most of the sweet or fruity ones have lemon juice in though so I avoided those but you're avoiding fruit anyway.
So this is what I would put out daily. For extras/treats you can give a bit of porridge or scrambled egg on a teaspoon and they just nibble it off the spoon. They love anything fresh or cooked. You mentioned oats so by making them into a bit of porridge it can be a bit easier for them. Latterly I bought the Ellas kitchen strawberry baby porridge - it's really easy and quick to mix and they go potty for it. Doesn't have fruit - just strawberry flavoured. And at this age anything they enjoy makes them happy
I have never heard of hamsters beign given Mirtazopine! I know for humans that is an antidepressant. But it also has quite a lot of side effects in humans - so keep an eye on things. Drowsiness usually. What does it say on the bottle?
I personally wouldn't given anything for his appetite - if he wants to eat he will - if he doesn't that is a natural sign that he is getting to his time of not wanting to try any more. And I think I would query the mirtazipine.
Keeping them happy and comfortable is the main thing. And still some out of cage cuddles. I used to give the porridge on a teaspoon during out of cage time.