Separate the feeling from the claim
Buying after a new high can feel uncomfortable. That feeling is different from evidence that waiting produces a better outcome.
The research question is whether a defined waiting rule improves outcomes after costs and missed opportunities.
Define a fair test
Specify the market, period, reinvestment assumptions, and the exact rule before looking at results.
A finished article would compare the rule against investing on schedule, with both favourable and unfavourable examples.