Look at the MC1377P IC from Motorola. This is an old NTSC/PAL encoder chip from the early 1980s but still is a good choice. To do S video, simply don't combine the chroma back with the lumance. Maxin and Analog Devices make some great video OPAMPS to use a line drivers.
There are also digital solutions but they would reacquire you convert the RGB to digital, although there are chips sets for that too.
Also keep in mind that broadcast and professional grade obsolete NTSC video gear can be found dirt cheap on Ebay. It might not be worth the trouble to DIY this in 2017.