Set a Dutch oven over medium heat and add the chopped bacon. Cook, stirring occasionally, for 6 to 8 minutes until crisp and the fat has rendered. Remove the bacon with a slotted spoon and set aside on a paper-towel-lined plate, leaving the fat in the pot.
Add the butter to the bacon fat and melt over medium heat. Stir in the diced onion and celery and cook for 5 to 7 minutes until soft and translucent but not browned. Add the garlic and cook for 1 more minute until fragrant.
Sprinkle the flour evenly over the vegetables and stir constantly with a wooden spoon for 2 to 3 minutes. The mixture will look like wet sand and smell toasty, which cooks out the raw flour taste.
Slowly pour in the bottled clam juice while whisking, then whisk in the reserved juice from the canned clams and the water. Keep whisking until the roux fully dissolves and no lumps remain, scraping the bottom corners of the pot.
Add the cubed potatoes, bay leaf, and thyme. Bring to a gentle boil, then reduce to a steady simmer. Cook uncovered for 15 to 18 minutes until the potatoes are fork-tender.
Press a few cooked potato cubes against the side of the pot with a masher or fork, then stir them back in to release their starch and thicken the broth naturally. For a smoother result, give the pot two short pulses with an immersion blender, leaving plenty of chunks.
Lower the heat to the gentlest simmer. Pour in the heavy cream and add the drained chopped clams. Warm through for 3 to 5 minutes, stirring gently, without letting it boil so the cream does not break and the clams do not toughen.
Remove and discard the bay leaf. Stir in the black pepper, then taste and add salt as needed. Ladle into bowls, scatter the reserved crisp bacon and chopped parsley on top, and serve with oyster crackers.