Can't wait to have mobile factories.

Assuming this ever gets finished.
There's a lot of unsolved problems with the treadmill thing.
Starting with the fact that entities are essentially duplicated.

I was thinking about replacing them with blueprint copies on the interior, but if there's going to be a charge time to deploy, and preventing movement while deployed, i think there should be a way to quickly toggle which surface your deployment is on.

I was also thinking you shouldn't be able to actually leave the tank and walk around on the planet, but perhaps that can be an option?

Right now it's just a proof of concept, please let me know what you think or where new features can be added, or existing features be changed entirely.

Do i really need the treadmill?

Interesting proof of concept. Duplication of the area around the tank on the interior surface is a very novel feature, but I agree that avoiding duplication of resources/etc. is needed.

New Force & Object (Unminable/indestructible) Ghosts: Let's say you have an ore field with a few miners on it and some belts. For the interior space, these items should be duplicated, as Ghosts objects belonging to a different force than that of the player so they cannot be interacted with. Similarly, other objects (rocks, trees) could be handled in a similar fashion.

Ramscoop: Driving over objects (rocks, trees) could auto-mine them (like the mining vehicles from AAI) and put their contents into an interior chest. Might be good as a default option (or an early/cheep upgrade) to add wood/coal directly to the fuel of the tank.

Deployable loaders: When the player stops driving, perhaps have a button to 'align' the tank to the nearest cardinal (N/S or E/W) so that loaders can be deployed from the sides, to transfer material in/out.

I want it to interact with both the planet and the harvester interior at the same time.
This does not appear to be possible (or rather - it IS possible, just not very UPS friendly by cloning all entities every frame).

The tank itself is not a chest so alignment is not needed.
A preview of the deployment outline would be better.

Take a look at these mods:
Subterra (https://mods.factorio.com/mod/subterra) for ideas on using multiple layers
Bob's Character Classes (https://mods.factorio.com/mod/bobclasses) for changing the player avatar.
Factorissimo2 (https://mods.factorio.com/mod/Factorissimo2) for creating and nesting factory interiors.

Personally,I think Alluvian has a really great idea. You pilot the tank across the map, harvesting EVERYTHING in your path.
Think "The Leveller" from the old animated film "Ferngully"
Your entire goal is to gather enough materials before the biter attacks get too serious and you hit the "Warp" button. But you are the one running around the map gobbling resources like Pac-Man eats dots.
Less transport logistics, More "get the automation built to run while you are driving"

1) The "Tank"
Have the character themselves be the tank. Non-removable power armor and weapons, with ammo being transported into the character's ammo inventory from the interior layers.

2) Inside the Vehicle
Have four levels "inside the tank".
When you enter the tank from "outside", you would be on the "upper deck", which would be the platform you build your defense structures on.
Have a portion of your script store how many turrets are crafted, and the number of each.
Export that to the "Character" tank on the surface of the planet as individual PA modules of "Auto Fire Weapons"
Take half that number and auto-equip an invisible gun of that "Level" ... 30 gun turrets would mean a "Turret x15" as a weapon. Make the ammo drain comparable, as well as the damage.
Have the second level be the "Factory" level. This is where you build the actual factory for building everything not related to fuel delivery.
Third level could be the "Reactor Level" Power generation for the vehicle goes on here. Think massive fuel tank, then match that fuel and amount to the exterior vehicle avatar.
Fourth level is "Infeed" and has a central area that is an exact clone of some portion of the area around the vehicle on the surface. The vehicle automatically "eats" entire resource patches (the script takes the resource in that grid square, applies it to the storage entity on the Infeed level, then removes the resource entity from that grid square. Think a player with a mining speed of 10M.

This is the dangerous level, because the vehicle is not moving while you are "inside" .... no one to drive it.
If you don't have the defenses to protect yourself from a Biter attack, they could (by the same process as you walk into the Factory in Factorrio or underground in Subterra) get inside and attack you.

Just some ideas I came up with looking over your mod.
Good Luck and I hope I see a Beta of this soon!