After being able to access The Sands Beyond head west along the southern edge of the map to the area with large spikes in front of a dumpster. If you stay on the edge of the cliff and move fast, you can avoid Pagurus.
Bobbit worms are marked with either a large rounded "X" in the sand or a steady stream of bubbles (see image). The Contact Lens isn't needed, but it will show their health bars and make it a lot easier.

Approach the worms to the side while hiding in your shell. This way if you are too close when they trigger you will be knocked back without injury. After a few points into defense their glancing attacks won't harm your shell either.
Be sure not to approach the center of the worm, as they will grab you and take your shell (this does not trigger the Kintsugi skill). Early game it may take a charged attack followed by a regular attack, but as you up your attack stat it can take just one charged attack or even a Reposte if timed correctly.

After gaining around 50 MSG (or using a Whelk stowaway) you can use a shell with Shards power to instantly kill them when they pop out. Paired with the Circle of Life skill you can run from worm to worm and never need to recharge your umami.
Crabs in the area can also be dealt with with shards at a high enough MSG stat along with the Circle of Life skill. If timed and positioned correctly you can take out the worm and a nearby crab at the same time.

Along this path you will also find 3 bottle cap crabs, 2 shot glass crabs, 2 thimble crabs, and a hermit crab hiding in a pop can.
Attacking the hermit crab will trigger it's reposte, so use an indirect attack (such as the Spearfishing skill, which has the the chance to have it clip through the ground).
When you reach the top of the hill grab the Bobbit Trap adaptation. The worms have the chance to drop Sand Dollars, Another Crabs, and other sellable junk and the hermit crab can drop Shark Eggs.

There are optimal paths to take, here is mine, although it is developed more through repetition than serious refining.
Upon reaching either end of the path, return to the title menu and reload the map to respawn all enemies and repeat the run.
Going up the hill, returning to title, and going down the hill earns 50k-60k microplastics, and each Fruit Sticker adds 15% (30% for Fruit Sticker+)
Worms are the X's (bubble worms are outlined). There are 35 worms total, but only 34 are reachable as one is underneath some rocks. The hermit crab is marked with the red exclamation mark.