The character must be at least level 17 Transmutation Wizard (to get True Polymorph) and level 2 Artificer (for infusions).
Main Loop:
- Use the Replicate Magic Item infusion to create the common magic item "Pot of Awakening".
- After 30 days, get an Awakened Shrub (small plant creature).
- True Polymorph it into an object no larger than small size.
- Profit.
Bare minimum to be strictly RAW, we need to start with an arcane focus.
Each Artificer infusion can only be applied to one object. However, there is some debate regarding whether one can pick the same infusion multiple times for Infusions Known. We'll pick the "Replicate Magic Item - Pot of Awakening" infusion as many times as possible to grow them in parallel.
Pot of Awakening
Wondrous item, common
If you plant an ordinary shrub in this 10-pound clay pot and let it grow for 30 days, the shrub magically transforms into an awakened shrub at the end of that time. When the shrub awakens, its roots break the pot, destroying it.
The awakened shrub is friendly toward you. Absent commands from you, it does nothing.
Replicate Magic Item technically needs us to supply the base object as it mentions:
In the tables, an item's entry tells you whether the item requires attunement. See the item's description in the Dungeon Master's Guide for more information about it, including the type of object required for its making.
Thus, we need to get an ordinary shrub and a clay pot for each cycle. Taking inspiration from How to polymorph one object into another, specifically a pumpkin into a carriage?, we will use the Major Transformation effect from the 14th level Master Transmuter feature:
Master Transmuter
Starting at 14th level, you can use your action to consume the reserve of transmutation magic stored within your transmuter's stone in a single burst. When you do so, choose one of the following effects. Your transmuter's stone is destroyed and can't be remade until you finish a long rest.
Major Transformation: You can transmute one nonmagical object—no larger than a 5-foot cube—into another nonmagical object of similar size and mass and of equal or lesser value. You must spend 10 minutes handling the object to transform it.
I'm not sure how a mass of material will be treated vs a single object (and whether the Wall of Stone counts as one object or you can split it into chunks), but you could either directly transmute into pots, or get a bunch of clay and use Potter's Tools to craft the pots.
The 3rd level artificer feature "The Right Tool for the Job" will let us create Potter's Tools out of nothing if we have thieves' tools/artisan's tools at the start. If the DM allows using an infused item instead of tools for this feature (like how it works for spellcasting), then you can start with just the arcane focus and infuse it to become an Enhanced Arcane Focus.
Similarly we get shrubs and plant them into the infused Pot of Awakening. Based on Are plants creatures or objects, the ordinary shrub should count as an object while the Awakened Shrub is a creature that is valid for True Polymorph.
We won't be able to learn Create Food and Water as it is a 3rd level Artificer spell, but 45 pounds of food seems like a good target for Major Transformation. I think water is generally not considered an object, so the lower level Create or Destroy Water might not be suitable.
I didn't consider the Awaken spell because it requires 1000gp consumable material, but it opens up the option of a Awakened Tree (Huge plant) which opens up more True Polymorph options maybe.
Enlarge/Reduce works on creatures and objects, but as a temporary effect I doubt it would be affect True Polymorph or result in free increase of a given resource.