Farming is work! Using magic can do a lot to improve efficiency, much like industrialization can in non-magical ways. I've divided the most relevant spells I could think of into some broad categories of farm work:
Field and Equipment Preparation
Fabricate (4th level spell)
Create tools and equipment
In some cases harvest and process in one step
Move Earth (...
A starting score of 18, +2 bonus from race, 5 improvements as you level up (4th, 8th, 12th, 16th, and 20th), a +6 enhancement bonus from a magic item, and a +5 inherent bonus from wish or a manual/tome, results in a score of 36.
This is a fairly typical “all-in” ability score at 20th level. Depending on race, class, and the particular ability score you’re ...
Though KRyan's answer is absolutely correct, I'd like to elaborate on having all three stats at 30. Just for completeness. It may be impractical but it is definitely not impossible. Magic items with +6 enhancement bonus would cost you 36,000 gp per stat (Periapt of Wisdom, Headband of Intellect and Cloak of Charisma), +5 inherent bonus would be either 25,000 ...
For this we will need a Luck Blade with at least one charge.
School of Necromancy Wizard 14
Find Familiar: 1st level spell
Your familiar acts independently of you, but it always obeys your commands.
Create Homunculus: 6th level spell
It is your faithful companion, and it dies if you die.
A homunculus is a construct that acts as an extension of its ...
Four (or 54 depending on how you interpret the Rod of Absorption)
A 20th level warlock has 4 Pact Magic spell slots, and there do not exist any items that give extra spell slots. See here and here for DNDBeyond searches that show all items containing "spell slot" and "spell slots".
That said, the Rod of Absorption can create spell slots ...
You probably want to be an Uttercold Assault Necromancer.
From K's Revised Necromancer Handbook -
The ultimate goal is to cast spells with the Cold Subtype that do half negative energy damage, while you and all your undead minions are immune to cold and healed by negative energy damage. Basically, this is done with Energy Substitution[cold] (a prereq for ...
A few more options, mostly similar to the ones in the question:
Smoking weapon enhancement (Lords of Darkness, p. 180): +1 cost equivalent. Makes a tiny Stinking Cloud effect in your space. Nauseated is a very good condition to apply (prevents taking most actions, including escaping a grapple), and you're already investing in save debuffs. Usefulness ...