I find real life and the internet to be two different things when it comes to bits.
In real life, tom thumbs and their cousins (argentine snaffles, "training snaffles," etc.) are considered pretty mild bits and the horses like them really well. I ride my Mustang in something similar and probably have a collection of about 10 of these bits with different length shanks, different looks, etc. They are my favorite bit. If I could only own one bit to use on every horse I ride, this would be it.
However, on the internet, there seems to be this notion (mainly due to one article written by a trainer) that these are awful bits, are too strong, and confusing to the horse. Well, all I know is, that I have only ever ridden one horse that didn't ride well in a tom thumb. I swear I can use it on just about any horse with great results. Of course I have light hands and ride with little contact.
So I say if you have gentle hands and the horse does well in it, use what the horse goes well in.
(But, I know most of this forum will likely disagree with me). I feel like the "Defender of the Tom Thumb." An important PS.
Your curb chain in attached in the wrong place. It should be fastened to the same rings that your headstall is attached. The big rings that you currently have your curb chain on are there in case you want to attach a rein as a snaffle with no leverage at all. It is not meant to be a curb chain attachment.
I have a feeling the curb chain is either too low or completely ineffective in the place it is now.