But it is reasonable that a large number could be eliminated over the course of one or two terms as president.

"Could be?" Sure. But it would never happen - because most federal employees are doing jobs that most people approve of.

The largest federal employers, by a mile and more, are the defense agencies (Veteran's Affairs, Army, Navy, Air Force, Homeland Security, DoD proper). Percentages vary, depending on whether you look at civilian vs. all employment (about 70% of federal employees are in those agencies, and about 36% of civilian employees are) and whether you include the Post Office in these discussions (USPS has about 600K employees, which is enormous). These figures are from 2019, but won't have changed much. The largest single federal agency by civilian employment is either USPS (if you count it) or Veteran's Affairs (if you don't).

ourpublicservice.org - FedFigures Workforce

It's hard to imagine Republicans being willing to take an axe - or even a scalpel - to those defense-related agencies. Indeed, there's a huge contingent in the party that has been pressing to increase defense funding, and presumably wouldn't take too kindly to RIF measures depleting their overall employment levels. Most of the agencies that Republicans/conservatives generally disfavor (HUD, Labor, Education, certain parts of HHS) just don't have that many employees, relative to the overall size of government.