There is still a simple and better way to improve the Gregorian calendar by not making leap years the century years divisible by 3200. Such improved calendar would have the average calendar year (365.2425 * 3200 - 1)/3200 = 365.2425 - 1/3200 = 365.2421875 days
and it would gain only
(365.24220 - 365.2421875)·1000 = 0.0125 days/millenium = 18 minutes/millenium
to the solar millenium and a one full day to 80 solar millenia:
(365.24220 - 365.2421875)·80000 = 1 day/80 millenia