Note that Cambridge has HSPS, not PPE, and that you can only apply to either Ox or Bridge (not both), and that additional written work and/or admissions exams are required for either.
UK won’t care about your ethnicity.
None of your ECs (except Young Republicans) will be at all interesting / relevant / considered. Young Republicans will be interesting only to the extent that it shows a generic interest in politics.
SAT “around 1470” and “probably” perfect score on essay implies that you haven’t taken it yet. 1470 is the minimum for Oxford. Cambridge wants a “high” SAT. Neither Cambridge nor LSE look at subject tests.
If your APs run as you expect, UCL / KCL will likely find your stats enough for an offer. Any offer from LSE and OxBridge will almost certainly be conditional on your CalcBC result.
There is no way to game Oxbridge: if you get all those 5s this year, get a good TSA score have a strong PS an interview offer seems reasonable, but after that the interview is make or break.
If you have the choice, AP Comp Gov is actually useful for the 1st year politics modules.