What's the benefit of balanced tonearm cables?

I can clearly hear the differences in balanced cable....are you saying that balanced cables sound alike?

They will if set up correctly. You might want to take a look at the Audio Engineering Society file 48 which defines the balanced line standard. I the shield gets driven by the source you will experience cable artifact- IOW various cables will sound different, although not nearly as much as with single-ended setups.

But eliminate cable effects? That’s a bit of a stretch for me. It does not cancel out capacitance, inductance or resistance.

Its not a matter of cancellation- its a matter of reduced IMD and swamped cable artifact due to low impedance operation. In the case of a phono cartridge, the source impedance is quite low, maybe only a few ohms, often terminated at the other end by a low impedance as well. Its hard for a cable to express any artifact of capacitance or inductance in such circumstances.

your bat gear is a balanced design and would benefit from balanced cables. Many companies simply provide the option for balanced or single- ended at the rear to accommodate customers (pseudo or fake). generally a balanced design will sound better running balanced cables-my gear has always been single-ended and sounds better that way. many of my friends talk about the increase in volume they get with balanced but do not focus of the sound quality difference if any. it was originally designed for longer runs in studios etc.

From the phono amp designers I have spoken to over the years it's not a simple case of one design being better than the other, but a question of implementation.
I recall one designer saying that he gave a balanced option to satisfy the demands of the high end buyers i nthe states and far east. He said that introducing the extra circuitry for the grounding in a balanced set up caused of itself problems, and wrongly done can cause sound.
Two of the greatest stages of all time were un-balanced - Vendetta and Mares/Connoisseur.

Balanced signals have greatly reduced common mode noise,

Just to be precise, balanced signals can have common mode noise, maybe even a lot. It's that balanced circuits reject the common mode noise and only amplify the differential signal.

How do I convert a single-ended tonearm cable, with the ground wire separate (2 RCAs per channel plus a thin ground cable) to a balanced tonearm cable using two male XLRs? I use an Ortofon Kontrapunkt b MC 'low output' cartridge.

The tonearm cable is ’integral’ with the tonearm, so I can work on the output end of the tonearm cable only.

The phono preamp has a true balanced circuit for MC cartridges using properly wired XLR male connectors from the tonearm cable. The preamp does have RCA inputs for MC (and MM ) cartridges. If I use the RCA inputs then it 'converts' the MC to single ended at the input only. If I use XLRs in a balanced fashion at the MC XLR input then the entire signal chain is balanced. The circuit is truly balanced.  I have seen the block diagram.

Thank you in advance for your advice on sewing this cable.