Unless you are actually going into the cart repair business as a source of income, all you really need to maintain your own cart is a DMM (Digital Multimeter).
Even then, the DMM doesn't have to be an expensive one.
Between the Voltmeter functions and to a lesser extent, the Ohmmeter functions, you can troubleshoot the cart's entire electrical system, including batteries and charger.
There is nothing wrong with your load tester, the problem is that the chart accompanying it is set up for car batteries (SLI - Start/Lights/Ignition) rather than deep cycle batteries.
On the other hand, using your cart as the load and a voltmeter, you can load test your cart batteries and discover their health.
Along the same line of reasoning, about the only thing you can find out with a hydrometer that you cannot determine with a voltmeter is which cell in a battery is bad.
Since you cannot replace just one cell in a battery, does knowing which one of the three (or four or six) cells in the sealed box is bad, do anything productive?
(Technically, if you can access the conductors connecting the cells together, you can determine which cell is bad using a voltmeter.)
To each their own, but I prefer to let the people who repair carts for a living buy the $pecialized test equipment and rent it along with their time when I need more than a DMM.
Added: To be technically correct, a single cell or cells can be replaced in a battery, it just isn't economically feasible with small batteries like those used in carts and cars.
In bathtub sized batteries costing thousands of dollar each, replacing a bad cell is a common practice. In fact, in some applications, the cells are stand alone units.