A useful personal system is rarely the most ambitious one. It is the one that leaves enough context behind to survive interruption, restart without drama and continue working when motivation is ordinary.
The question behind write decisions down before the context disappears is what changes in practice. The useful answer is usually found between capture the question and keep the note near the work: the point where a broad idea becomes a choice that can be made, observed and revised.
Write what needed to be decided and what constraint made the decision necessary. This keeps future readers from treating the result as a universal preference.
A short note about the serious options and why they were rejected is often more valuable than a long description of the winner. It stops the same investigation restarting from zero.
State what would change the decision
Capacity, cost, product direction or available tooling may move later. Naming those triggers makes it easier to revisit the choice without pretending the earlier work was wrong.
Use a format that will still be found: repository documentation, an issue, a runbook or a small decision record. A perfect note in an invisible location has little value.
The best result is a cheaper restart. When the current state, next action and reason for a decision remain visible, a disrupted week becomes a pause instead of a reset.
That is the standard I would use when returning to write decisions down before the context disappears: keep the reason visible, make the next decision small enough to understand, and leave enough evidence to know whether the approach is still helping.