I’m very familiar with FCPX it’s the only editor I use, I just bought a
M3.
Tell the timeline 50fps even tell it 60fps which is a native setting. The timeline simply wants to know the highest frame rate and res any clip you add might be? If you add a clip at 50 it won’t care if you set it to 60 FPS.
When you slow it down to 25 you may notice it looks a bit jumpy. No worries that can be fixed some buy using frame blender or optical flow. Optical flow yields big files and takes longer to render, and sometimes frame blender does work better, try both.
And remember, the harder you turn, or fly fast near objects the more likely the video will appear jumpy when slowed down. If you plan to turn fast or fly quickly near objects use 4K at 120 FPS. If flying high and not near anything just doing broad range landscapes your 5k 50fps should work well for you even after slowing it down.
Good luck