Both of these questions are very dependent on the Universities that you’re targeting.
Most top schools require the optional essay. You get extra time for the essay, so I don’t see too many reasons not to do it - plus, it’s probably the easiest part of the SAT! Some places won’t need the SAT, and some places are becoming test-flexible too, but I’d definitely recommend doing it if you want to go to any of the top schools.
As for subject tests, some Universities don’t even require them - however, if your university needs it, or if you want to demonstrate a particular academic strength, definitely take them. For me, I took the core SAT and three subject tests. Stanford doesn’t require subject tests, but I wanted to demonstrate my strengths by taking Literature, Physics and Maths II.
If you want University specific advice, feel free to ask! Otherwise, have a look around on their admissions pages for more details