I might be the only person on Steam who has acquired this achievement, so I want to explain how to achieve this with to no effort. Completionist is really the only hard achievement and it's more a matter of setting up the achievement and leaving it running while you're at work, school, sleeping etc. Basically you are going to acquire a ton of (100-150) Gourdlets, level them to maturity using environmental exploits, wait several hours, get them to "pop" by hovering the mouse over them, order the next parcel train item and repeat the process.

The Gourdlets are basically mindless NPCs that interact with everything in front of them without preference. They also seem to level at the EXACT SAME pace regardless of whether they are brand new to the town or have been there since the beginning. You can actually monitor the progress bar by creating a new Gourdlet and looking at the level it has advanced in 30, 60 or 90 minutes- if all other Gourdlets have equal access to interactions they should all average out to progress at nearly the same rate.

The key to unlocking all the parcel train items is volume- so at the beginning create 100-150 Gourdlets. This is tedious because you have to keep calling the train to invite new ones but eventually you should have 100-150 wandering around the map. The Gourdlets will never actually enter the buildings so you don't even have to bother building them. The main things you want to create are piers and hedges, as they will interact nonstop with these.

The easiest way to ensure they are always leveling is to create a 4 x ??? block of hedges and pick up your Gourdlets and trap them in there for all eternity. The width should be four so they don't overlap when they chop the hedges on each side, but the length can be anything. You don't HAVE to do this but it makes monitoring their behaviors and ensuring they are being milked for every interaction. If they're near a pier they will also go fishing, so if you create an open plan just put piers on all sides. Forcing them to swim by picking them up and placing them in the water doesn't appear to count as an interaction. When you go to "farm" your Gourdlets for levels simply delete all of the hedges so you can be sure none of them are hidden by the environment.

As you unlock more parcel train items, it takes longer and longer to level. The final three levels took me about 10 hours of passively running the game in the background. Sometimes quitting and restarting also adds levels to the bar, but I'm not sure why. Once you've leveled a Gourdlet hover your mouse over it and the bar should increase and the level of the Gourdlet resets. If you create all of them at the same time they should all approximately level at the same time, so get them all next to a pier or trap them in a hedge maze, tab out and wait. Just keep in mind the game WILL lag even if you have a lot of RAM, processor speed and a good video card. It's OK if the game lags because even if it crashes the game autosaves periodically. Remember, since they all level at the same time the key is to create as many as possible at the start and get them all reacting to things immediately.

It will take about 30 hours of passively running the game but you should get there and you will finally be at 100% completion, assuming you got all the other achievements. And also you'll have this nice statue to spam all over the map.