bqb ZWO also recommends total counterweight no heavier than 5kg. So...does this mean for every kg over 10, I should use 0.5kg of counterweight?
Perhaps not -- keep in mind that the counterweight adds to the stress on the bearings that maintains the latitiude angle of the mount. That 5 kg (added to the mount's payload) number may indicate just that safety limit.
The rule of thumb with the strain wave geared mounts before ZWO's mount is that you only need to add enough counterweight to keep your tripod from tipping over for all possible OTA positions.
To prevent the tripod from tipping over, it is also often better to weigh the center of the tripod down with a sandbag -- which also adds to the overall stability in a breeze -- i.e., place a weight somewhere else instead of on a counterweight shaft. Gitzo carbon fiber tripods, for example, come with a hook to add such a weight. You can also buy aftermarket weight bags like these:
https://www.amazon.com/Selens-Universal-Tripod-Stone-Bag/dp/B01GQ0FSZO
https://www.amazon.com/Vanguard-SB-100-Stone-Bag/dp/B0051OGJQE
In my case (an RST-135) I went from placing a counterweight (a counterweight from a Takahashi mount) here:
to placing them (and a brick) at a better location here at the spreader of the same William Optics Mortar tri-pier:
Chen
