This is a low-confidence partial answer. But perhaps it will assist someone in formulating a more complete and correct answer. Or maybe it will help jog the OP's memory if they return.
Are you remembering The Amazing 3 (also called Wonder 3, or W3, original title ワンダースリー)?
The Amazing 3 was a 52-episode series created by the "Godfather of Manga" himself, Osamu Tezuka (creator of Astro Boy, Kimba the White Lion, Black Jack and many, many others). The Amazing 3 was dubbed into English and aired in the USA and Australia starting in 1969, and probably rerun in later years. (But was the dub ever on VHS? Not sure. Original Japanese-language definitely was.)
On to the evidence...
RE: "The little guy evades, reaches his mono-wheel kind of UFO and drives off."
There are several monowheels in animation, but the third image in the question is an absolute ringer (pun intended) for the Big Wheel, a vehicle used by the protagonists quite often throughout The Amazing 3, and can be seen in the end credits of every episode.
The protagonists are often depicted peaking out the hemispherical windows as it speeds along (on land or even water) as drawn by OP.
RE: "All I remember is the hero, maybe a little green alien kid, being outside on a green field of grass, maybe a farm."
One of the primary protagonists is a human boy, shown in one of the screenshots below wielding an axe.
His parents live on a farm, shown often throughout the series. Backgrounds often depict pastoral settings with rolling hills.
The boy adventures with the titular Amazing 3, who are aliens sent to Earth to evaluate the planet's inhabitants' prospects for peace. Their alien forms are shown in the intro of many episodes, but throughout the series they are mostly shown in their adopted Earth forms of an anthropomorphic rabbit, horse, and duck.
RE: "I think it has something with a "K" in it, like Kirk or Kirby."
In the English dub, the human boy's name is Kenny.
Alternatively, if you actually saw the Japanese-language version subtitled in English, the three alien protagonists are named Bokko, Nokko, and Pukko.
RE: "And there's some kind of slug or worm out there underground, trying to kill him. Its piercing its spikes through the ground. [...] The spiky slug then comes to the surface and you can see its giant form."
Here, unfortunately, is where the evidence peters out. Skimming through the episodes, all of which are currently available on YouTube, I was unable to spot a giant slug creature with spikes as drawn in OP's first and second sketches.
The closest matches I could find were a giant snake and a plant creature with pointy tendrils that shot up from the ground, both from "The Mystery of Shangri La" (episode 3).
Conclusion
Since these creatures don't match the OP's giant slug, but the Big Wheel matches quite well, here are some possibilities:
The memory in question is The Amazing 3 (misremembered in color) but a different episode I have missed.
The memory in question is a combined memory of The Amazing 3 and something else.
The memory in question is of characters from The Amazing 3 crossing over into another show, most likely another of Tezuka's anime, which definitely happened occasionally.
The memory in question is of a different show that used a vehicle identical or similar to the Big Wheel as an homage to Tezuka.
Coincidence.
