If I can offer some comments, I’m very familiar with K5s and with B5s. The Carvin has tight string spacing and a narrower neck. Very comfortable if you have small hands. The Kingston has full string spacing, an although the neck is very thin and still very comfortable, the B5s generally is easier to play. Unless you have large hands and/or long fingers, then the K5 will feel like home.
Both basses balance perfectly and are lightweight. The K5 being 35” scale, will have a better low B. The Carvin’s low B is okay, very usable, but can’t match the clarity of the MTD.
Build quality will be better on the Carvin, but the Kingston is not bad for the price. The fretwork is great on both.
Lastyy, the MTD will give you a Musicman-type sound, whereas the Carvin will have the MM sound, plus a Jazz-Bass-type sound, depending on whether you have the MM/J configuration, or the Jazz configuration. A little more versatility on the Carvin.
I personally would stay away from SX. They look okay on the computer monitor, and the prices are enticing, but they need a lot of setting up and upgrading to make them playable basses, and if I may speak freely, they are nothing more than Chinese crap. (My opinion and experience, of course, nobody take offense, please).
Hope this helps.