I went into Tin Cheung which, as you say is in a small mall (upstairs on perhaps the 3rd level) near Nathan Road. It has a reputation for being reputable and looked very professional but I did not buy from there on my last trip.
As you have said many stores in Tim Sha Tsui (TST) are tourist traps. In fact they are traps of the worst kind where they regularly try to bait and switch customers. You ask the price. They will give you a price that sounds too good to be true, which of course it is. When you offer to buy they will take your money then say they have to get the item "from the warehouse" They make a big show of calling the "warehouse" only then to say very sorry but we are out of stock and instead try to force upon you, some piece of crud worth half what you paid and they refuse to refund your money. The police will not get involved regarding it as a civil matter. Even if there is one of what you bought on the display in the store they will not sell it, claiming its a display item only. This is very common or was when I was there about 2 years ago but I was spared problems as I ad researched it and was savvy enough to be careful. After making due inquiries I bought a lens from a small dealer on the corner of Hankow Road and Ichang Street in TST and was very satisfied with their service and no bullsh#t policy. ( A good test is the price quoted. If its in the ball park of what you expected to pay there is a chance they may be honest. If the price quoted is silly cheap you can be sure they are crooks. Many times I asked just for curiosity then just turned and walked out without a further word when this happened to me).
You will also find an interesting camera store loaded with older and collectible cameras in a little shopping mall (Mirror Tower) opposite the Shangrila Hotel on Mody Street in TST about a 10 minute walk from Nathan Road. If you are looking wider afield (as others say Mongkok is good including Sim City and various shops in Sham Shui Po). Check out the videos of Matt Granger on youtube (He was also known as That Nikon Guy so you may fiond that helps when searching) He has three or four good videos on shopping and photography in Hong Kong and his videos tell you about his favorite stores in Mongkok. Here is one (Google Matt Granger Hong Kong video)
https://www.youtube.com/watch?v=Rp6jXyia-Pk
Be aware that shopping in HK is not as good as in pre internet days and with rising incomes H
ong Kongers can be willing to pay big money for collectable camera gear..