How about full character based circles using Bresenham’s circle drawing algorithm in Sinclair BASIC?

Thanks to @GoryDetail for pointing me in the direction of this algorithm

https://www.geeksforgeeks.org/bresenhams-circle-drawing-algorithm/

Source available at https://github.com/jimblimey/random-stuff/blob/master/rcircles.bas

Because I said I'd give the ZX81 more attention, here's the code running on that beauty