Radial fauna is most commonly found in sea life. I encourage you to check out the gorgeous Kunstformen der Natur for gloriously etched examples. Many of these are microscopic in scale (i.e. radiolarians, diatoms) but larger radially symmetrical organisms or multi-organism structures exist, like jellyfish and corals.
https://upload.wikimedia.org/wikipedia/commons/b/bf/Haeckel_Diatomea_4.jpg
The smaller the structures, the closer they tend to resemble geometric shapes exactly, so you may want to think about scaling from a three-way perspective of size, precision and total numbers (e.g. a sea surface of millions of microscopic but geometrically perfect plankton may generate a sort of fuzzy, undirected but perhaps powerful magic field).
To get powerful, directed and purposeful geometric magic you want radial symmetry, pattern-generating abilities and intelligent will, which brings us to…
Magic-wielding octopodes
The octopus body plan is already eight-fold symmetric. 3
Many octopuses (or your favourite plural) have the ability to change the pattern of their skin with exceptional control and precision, which they use for feats of camouflage that, as far as I’m concerned, already count as magic. They are also very, very clever. By shaping linear designs onto their arms, they would learn to generate magic patterns for hunting, protection etc. (Or potentially just to amuse themselves! Why not?) This doesn’t need anything evolutionarily beyond what they already have, and their learning abilities are already perfect to get them to experiment by trial and error (I seem to understand that your magic system is weaker for incomplete or imperfect forms, which would be great for experimenting as you could gradually sharpen your design and stop if you feel you’re catching fire or something)
Bonus: magic toadstools
Another group of radially symmetrical organisms, land-based this time, are the fruiting bodies of fungi. I’m not an expert in the least but I think, generally, extant mushrooms would be somewhat too simple for your requirements (mostly they have a pattern of straight radial gills underneath, and/or hexagonal textures on top like the bridal veil stinkhorn) but with a bit of selective pressure they may easily develop patterns for thaumaluminescence or spore-ejecting magic. 671