As far as the "initial" question posed by OP, if history repeats itself Republicans generally spend more money on defense and Democrats usually tend to spend more money on social programs. How that will effect the "bottom line" for the services I have no idea, but I doubt it will have any effect on manpower as the services still lose a considerable number of people upon expiration of contracted service, so they still need to recruit and commission X number of new people annually. Where I think, and this is JMHO, is that major programs such as acquisition and modernization may take a major hit.
Budgets are submitted by the President and approved by Congress, so the new President will have his say starting FY 2022 and we shall see if any changes are requested. As for personnel, JMHO but I think a Democratic Administration will roll back many or the more restrictive personnel policies of the current administration, but the country still needs to maintain military readiness to defend our country so the cuts can't be too Draconian!