My iPad is about 6 years old ios10.3.3 and doesn't seem to be a software upgrade available

I'm guessing that your iPad is the iPad 4. It cannot be updated beyond iOS 10.x.x. It does not have the components needed to run a newer version.

A six-year-old regular iPad isn't capable of running 11 or above; 10.3.4 is only available for models affected by the GPS issue it patches.

(187816)

Your iPad is not compatible for iOS 11 or later. The max you can go is 10.3.4.