I don't use any. What I do is create the world in 1.12.2, then I use mapwriter mod and zoom across the map until every area is explored. Then I upload it in 1.13.2, go and add the last touches including bonemealing areas to add sea weed and stuff, then it's done! The key is exploring every chunk of your map. If you don't, then when it's uploaded into 1.13.2 the chunks will be reset. The hardest part is actually going and adding 1.13.2 stuff, because bonemealing a map for seaweed is quite time consuming.

Yeah, sorry it's hard to see on the map. It's labeled "Landing Site".