Let's bump this thread once again.
5 months after tentative promise(1) and 3 months after explicit "sometime in May" assurance(2), there is still no official Corsair firmware update for MP510.
1) https://forum.corsair.com/v3/showpost.php?p=1000119&postcount=7
2) https://forum.corsair.com/v3/showpost.php?p=1002976&postcount=16
I'm not even that surprised. Though I ended up buying MP510, it was mostly because there was simply no option for me to buy other Phison E12 clone from a company providing functional customer support.
Nevertheless some lessons were learned here and I now know what to expect from Corsair. :)
And to be clear, using Phison updater to 12.3 did work fine for me. But it doesn't change the fact that Corsair decided that cutting costs on the support in expense of the customers is completely fine.