For the sake of context, this is a fantasy setting where this character has no access to magic and lives in a small village. He would like to write a book and since wiriting materials are a little expensive he would like to plan out pages in inexpensive chalk first. My thought is to use a series of unglazed clay or ceramic tiles since clay is plentiful. Does this make sense or would it be too much of a pain to use?
The ancient erasable and reusable low-tech scratchpad is a wax tablet. Wax tablets were used for a thousand years in the classical world, and they have the advantage of being hand-portable, unlike a blackboard.
Left, modern reconstruction of a Roman wax tablet diptych. One wrote on them with the sharp end of the stylus; corrections and erasures were made by tamping the wax with the blunt end of the stylus; for a full reset the wax was melted. Photograph by Toilet, available on Wikimedia under the GNU Free Documentation License, Version 1.2 or any later version. Right, a 19th century slate tablet for use by school pupils; one wrote on them with a chalk pencil, and they were erased with a damp sponge. Photograph by GodeNehler, available on Wikimedia under the Creative Commons Attribution-Share Alike 4.0 International license.
Blackboards or chalkboards are a modern invention, late 18th or early 19th century. The original blackboards were made of natural slate stone, sort-of oversized versions of the slate tablets used by school pupils since the 1500s or so.)
The Greco-Roman wax tablets and the early modern slate tablets have left their traces in the fixed phrases tabula rasa (literally, scraped tablet), (to start from a) clean slate, and to wipe the slate clean.
Birch bark, provided that you have those trees around, is rather cheap as a writing material:
Birch bark or birchbark is the bark of several Eurasian and North American birch trees of the genus Betula.
The strong and water-resistant cardboard-like bark can be easily cut, bent, and sewn, which has made it a valuable building, crafting, and writing material, since pre-historic times. Today, birch bark remains a popular type of wood for various handicrafts and arts.
Removing birch bark from live trees is harmful to tree health and should be avoided. Instead, it can be removed fairly easily from the trunk or branches of dead wood, by cutting a slit lengthwise through the bark and pulling or prying it away from the wood. The best time for collection is spring or early summer, as the bark is of better quality and most easily removed.
To prevent it from rolling up during storage, the bark should be spread open and kept pressed flat.
It was commonly used before the invention of paper
Birch bark manuscripts are documents written on pieces of the inner layer of birch bark, which was commonly used for writing before the advent of mass production of paper. Evidence of birch bark for writing goes back many centuries and in various cultures. The oldest such manuscripts are the numerous Gandhāran Buddhist texts from approximately the 1st century CE, from what is now Afghanistan.
It has also the advantage of taking less space than writing on pottery or tablets, because it's thinner.
Consider the metalpoint / https://en.wikipedia.org/wiki/Silverpoint technique. The wikipedia page gives a good historical account you can use for inspiration
Several folk have already mentioned bark as an erasable substrate. Paper covered in wax works as well, and is an upgrade. And access to graphite works event better
You could have a story line where the writer gradually upgrades materials - maybe using fishbone + bark at first, then getting access to metal pointer, finding chalk and/or graphite, with paper upgrading to wax on bark, then wax on hemp or other rough fibre, then paper etc...
Charcoal on panel.
Coat the panel with traditional gesso, a mixture of skin glue and chalk (or another whitening agent). The result can be a perfectly smooth and beautifully white surface, and you can easily wipe off the charcoal and start over. When the surface gets too grey, you can wipe it clean with a little water, or simply apply a new layer of gesso.
If you have pottery (very likely in medieval settings), then the ostraca are cheap (literally broken trash), plentiful and durable. And they were good enough for ancient Greeks as well.
Alternately, in more Chinese-like settings, where bamboo is abundant, you can use bamboo slips.