Critique & Input on my super basic religion concept

that actually sounds like a really cool idea. some ideas for names, just use latin, greek or gaelic words, i find they sound really fancy haha. or another thing i am thinking of doing is taking gods and goddesses from loads of different religions, changing them a little and making something else out of the whole group of things. it sounds weird but when done properly can be really cool

all good

Consider using the calendar/year to figure out when the hottest/brightest time and coldest/darkest time of the year is. These could be used as holidays for the light/dark gods and then the halfway points could be holidays of the goddess. For names I would think about:

Bryn, Brynnyg, etc. for the god of light
Shendar, Shen, etc. for the god of darkness
Khura, Khuri, etc. for the goddess of magic

They just seem like they fit.

For ceremonies you might have priests/priestesses doing a ceremony at each of the above holidays to turn the seasons, say good bye to one god and welcome in the other (or have them fight each other like the Oak King and Holly King), and honour the balance at those points.

Hope this helps!