I'd definitely go with Harry Hamster anywhere it is available,even if you must order it online!
Fiber is one of the less crucial nutrients,and getting it a bit low in a main diet isn't a big deal.You do want some,but nowhere near rabbit or guinea pig levels.Again,Harry Hamster is perfectly adequate
As for some extras that haven't been mentioned,it is fine for Syrians to occasionally have bits of dried fruit as a treat though this shouldn't be given more than once every 3-4 days if feeding small amounts.Be careful that there are no added ingredients like unsafe preservatives(mostly sulfur dioxide)or added sugar etc.Peanut/monkey nut is also a great nut option,and these can be a bit cheaper than some nuts but make sure to get plain peanuts with no salt.They can be roasted though!The main point is keeping it simple and natural instead of going for storebought junk,though there are some really nice storebought treats if you look hard enough