Id try composite shims, the wood ones will eventually mush under the weight. For a 1/4 I wouldnt even bother. When you can actually see he water line being un level you know its time to top off. My stand is on rubber mats though to protect my floor.
I think that stand you bought will hold it fine so long nothing whacks it too hard on the side. When I built mine, that was one of the tests I did. I literally kicked the side flat footed and NOTHING. To me the biggest threat of collapse is a side dropping, even more than the middle. I mean people put so much weight on the glass itself which is COMPLETELY unsupported. Rocks, wood, gravel, etc etc... all on unsupported glass. When I built mine I was going to cut a piece of ply just to support the glass but that worried me to with expansion and contraction pushing on the silicone.
I also have the luxury that my stand sits VERY low. All my stuff is low, I like it that way for watching while sitting and easy cleaning.
Those stands are manufactured to hold the weight, they sell them everywhere but look flimsy. Its like IKEA furniture. You build a TV stand and its wobbly, shaky, and ready to fall over. You then tack that horrid little piece of paper board to the back and you can now drive a truck on top to change your oil. Sad to say but most of these companies have people that know what theyre doing, they save a buck but know what theyre doing.
I would have just built one, I think you could have.
