[rolftheperson]

This is not correct, I urge you to read a bit about Lorentz invariance, once you understand you will see why your statement does not make sense given special relativity is accurate.

Lorentz invariance means the laws of physics remain the same in all inertial reference frames. Also a spaceship going 99.9999…% the speed of light.

This leads to effects like time dilation, length contraction and the speed of light itself.

The metronome can keep going at any speed independent on the speed of its own reference frame.

[jvanderbot]

Yes, and for that metronome to continue "ticking" according to the normal laws of physics the distance must shrink to accommodate the limits on speed - Length contraction.

But, all the observable effects are the same under my "life raft" mental model: To an outside observer their time has stopped, to an inside observer it has not (I'm just thinking slowly). And the laws of physics are unchanged - as a function of time e.g., the number of clockwork steps per metronome tick.

I realized that my thought experiment is not special or unique. Here's the "light clock" version

https://www.emc2-explained.info/The-Light-Clock/