On the proportional explanation you have half-rejected: it is worth knowing it is an old idea, that it does capture something, and that your objection to it is a good one.
The idea is that each interval is judged against the length of life so far, so a year at fifty is a fiftieth and at ten it is a tenth. It produces a curve that matches the general shape of what people report.
What it does not explain:
The lurches. It predicts smooth compression, and people report specific years as long or short.
Why a holiday feels long afterwards and short while it is happening, which is a well-known and slightly paradoxical pattern.
Why the effect can reverse. Retire, move country, or change everything, and people frequently report time slowing again - which a pure function of age cannot produce.
The encoding account handles all three, which is why it has largely displaced the proportional one as the main explanation. They are not incompatible; the proportional element may contribute.
The holiday paradox is the clearest demonstration that these are two different clocks: in the moment, a novel week is busy and passes quickly; looked back on, it is full of distinct memories and reads as long. Ordinary weeks do the opposite.