you got very lucky 'timing' the market this year, but that's just luck.
It's not clear by your post whether you mean, take the 'chunk' of money and just buy 1/12th into the market every month in 2019, or you mean, load your roth for 2018 in december 2018, and start contributing monthly starting in 2019. If it's the former, just do the lump sum, if it's the latter, why having you been doing it all along (ie you had spare $ every month, but waited until the end of the year when you get your 'chunk')