I'd definitely use Fitch rather than wood substrate.There are so many reasons why it's better,like being more suitable to burrowing without anything needing to get added plus it cannot cause an allergic reaction(surprisingly common!)and is not an injury risk either as wood can be.
As for how often to do full cleans,you shouldn't ever have to
It might sound crazy,but the only good reason for fully cleaning out substrate at once is if a hamster gets seriously ill or passes away and the cage needs to be sterilized before being used for another hamster.Just make sure the cleaning supplies used are all natural and entirely hamster safe,for example in those extreme cases such as illness a high quality enzyme based cleaner(no harmful additives of course,more on that in a moment)or in other situations a simple vinegar and water solution.In the every day scenarios however,simply removing substrate is more than enough.
Keep in mind harmful substances can be found in many products,whether it be cleaning products or treats or even food.These would be unspecified biproducts(such as digest,derivatives,meat and bone meal of any sort,biproduct meal,animal liver or generalized liver,animal fat,or any other unspecified or generalized ingredient),artificial flavors,artificial dyes(sometimes referred to as "E&C approved" in the UK,but either way these are not good to have in any product),artificial sweeteners or processed sugars(such as dextrose,maltodextrin,sorbitol,fructose,sucrose,et c),sulfur dioxide,unspecified preservatives or BHA/BHT/ethoxyquin(labelled "E&C approved antioxidants" and similar things in the UK,but no matter what these are not safe as they are known to cause cancer among other things),and propylene glycol.Of course,anything to be shared with dogs or cats must not have any corn or wheat(safe for hamsters,terrible for the mentioned animals!).Nothing should ever be made in China or irradiated either,regardless of species.