I've just recently finished a project with a lot of complex curves, but also partial circular sections. There were no complete circular holes, but lots of chords of circles and when I was laying it out, I wanted to work out the radius of the circle for a given chord length and chord height, so I that I could buy a few appropriate large forstner bits to create the holes before using a bandsaw to cut out the profile.
It turns out that for a chord of height h and length L, the radius of a circle is: R = 1/2h + L²/8h Here is an excel file
This worked well for choosing which bits I needed. For the complex curves, I found that the french curves that are used for drafting had the right shapes, but were the wrong size. So I created pdf files of the three most common Burmester series of French Curves that I could scale to the size I needed on a printer. I've included free copies of these pdf files in the useful woodworking links section of this website and I've also added links below. These pdfs can be printed out at various scales and sections of them cut out and pieced together to create curved profiles for project, or even glued to template material and cut out to create french curve templates at required scales.
On my project, to transfer the curves to the parts, I ended up buying a 24" flexible curve made by Staedtler Mars and using it to copy the curves from the printed french curve pdfs and then tracing the flexible curve. I highly recommend these flexible curves that are available at various art and drafting supply stores such as Staples https://www.staples.ca/en/Staedtler-Mars-Flex-Curve-24/product_13328_1-CA_1_20001, Michaels, Office Depot, and other Art Supply Stores - they are typically less than $10 and really are useful for projects with complex curves.