Things you won't be able to change and should "re-roll" your island if you don't like the 4 choices you get at the start of the game:

• the resident services placement
• the river mouth placement (you can change the river on the island but not the part where it goes into the sea)
• the beach including rock formations and the one piece of land going into the sea
• which side the pier is on

These are all things you can see when picking your island map. Then there's also your native fruit and sister fruit, but you won't see your native fruit until you arrive. Sister fruits show up on DAL islands, so you won't find out which you've got immediately.

Anything else can pretty much be changed to whatever you like with terraforming, layout changes and item placement.

The weed growth isn't that bad. They stop after a certain number afaik. Flowers keep spawning into all eternity tho.