Generally speaking, there’s no one “catch all” mix. Most of the time you will need to mix multiple foods together to get a good balance of nutrition and variety.
That being said, I’ve generally recommended to those in the EU a mix of these two:
https://www.zooplus.com/shop/rodents...er_food/621825
(Even though it’s labeled as a dwarf hamster food it’s great for syrians, too)
https://www.zooplus.com/shop/cats/ca..._treats/208657
(Freeze-dried chicken—I wouldn’t use boiled chicken as it has a higher water content and, in turn, less protein)
A mix of one of each container would get you to 18.64% protein, 6.43% fat, and 6.65% fiber. The fiber is a little low but you can add in grains like oats and barley to increase this
Let me know if I wasn’t clear on anything!
~Robin