The original idea for stardates was that they'd be tenths of a day. I always assumed that they were measuring the ship's time out of space-dock, that they weren't a calendar. I do wish that idea had survived, it was cool, simple and intuitive.
Subjective time out of spacedock is cool and handles any issues with relative time.
Well, it is a cool idea, but it wouldn't explain the stardates in consecutive episodes being out of order, since then they'd have to be going back into dock every week.
If you say that it takes X days in one episode at warp W, then it has to be that way always, or you have to suggest reasons why not, such as headwinds and tailwinds in space.
Well, they don't have
to, since only a tiny fraction of viewers are going to care enough to bother to remember the figures and do the math.
But the idea has been around since at least 1980's Star Trek Maps
that the relationship between warp factor and velocity is variable depending on the local conditions of space and/or subspace, such as mass distribution and the resulting spacetime curvature. That's actually stated outright in the TNG Technical Manual
and ST Encyclopedia
, though for some reason fandom seems to have mostly overlooked it.